The document at https://tc39.es/ecma262/ is the most accurate and
up-to-date ECMAScript specification. It contains the content of the most recent yearly snapshot plus any finished proposals (those that
have reached Stage 4 in the proposal process and thus are
implemented in several implementations and will be in the next practical revision) since that snapshot was
taken.
This specification is developed on GitHub with the help of the ECMAScript community. There are a number of ways
to contribute to the development of this specification:
Refer to the colophon for
more information on how this document is created.
Introduction
This Ecma Standard defines the ECMAScript 2026 Language. It is the seventeenth edition of the ECMAScript
Language Specification. Since publication of the first edition in 1997, ECMAScript has grown to be one of the
world's most widely used general-purpose programming languages. It is best known as the language embedded in web
browsers but has also been widely adopted for server and embedded applications.
ECMAScript is based on several originating technologies, the most well-known being JavaScript (Netscape) and
JScript (Microsoft). The language was invented by Brendan Eich at Netscape and first appeared in that company's
Navigator 2.0 browser. It has appeared in all subsequent browsers from Netscape and in all browsers from
Microsoft starting with Internet Explorer 3.0.
The development of the ECMAScript Language Specification started in November 1996. The first edition of this
Ecma Standard was adopted by the Ecma General Assembly of June 1997.
That Ecma Standard was submitted to ISO/IEC JTC 1 for adoption under the fast-track procedure, and approved as
international standard ISO/IEC 16262, in April 1998. The Ecma General Assembly of June 1998 approved the second
edition of ECMA-262 to keep it fully aligned with ISO/IEC 16262. Changes between the first and the second
edition are editorial in nature.
The third edition of the Standard introduced powerful regular expressions, better string handling, new control
statements, try/catch exception handling, tighter definition of errors, formatting for numeric output and minor
changes in anticipation of future language growth. The third edition of the ECMAScript standard was adopted by
the Ecma General Assembly of December 1999 and published as ISO/IEC 16262:2002 in June 2002.
After publication of the third edition, ECMAScript achieved massive adoption in conjunction with the World Wide
Web where it has become the programming language that is supported by essentially all web browsers. Significant
work was done to develop a fourth edition of ECMAScript. However, that work was not completed and not published
as the fourth edition of ECMAScript but some of it was incorporated into the development of the sixth edition.
The fifth edition of ECMAScript (published as ECMA-262 5th edition) codified de facto
interpretations of the language specification that have become common among browser implementations and added
support for new features that had emerged since the publication of the third edition. Such features include
accessor properties,
reflective creation and inspection of objects, program control of property attributes, additional array
manipulation functions, support for the JSON object encoding format, and a strict mode that provides enhanced
error checking and program security. The fifth edition was adopted by the Ecma General Assembly of December
2009.
The fifth edition was submitted to ISO/IEC JTC 1 for adoption under the fast-track procedure, and approved as
international standard ISO/IEC 16262:2011. Edition 5.1 of the ECMAScript Standard incorporated minor corrections
and is the same text as ISO/IEC 16262:2011. The 5.1 Edition was adopted by the Ecma General Assembly of June
2011.
Focused development of the sixth edition started in 2009, as the fifth edition was being prepared for
publication. However, this was preceded by significant experimentation and language enhancement design efforts
dating to the publication of the third edition in 1999. In a very real sense, the completion of the sixth
edition is the culmination of a fifteen year effort. The goals for this edition included providing better
support for large applications, library creation, and for use of ECMAScript as a compilation target for other
languages. Some of its major enhancements included modules, class declarations, lexical block scoping, iterators and
generators, promises for asynchronous programming, destructuring patterns, and proper tail calls. The ECMAScript
library of built-ins was expanded to support additional data abstractions including maps, sets, and arrays of
binary numeric values as well as additional support for Unicode supplementary characters in strings and regular
expressions. The built-ins were also made extensible via subclassing. The sixth edition provides the foundation
for regular, incremental language and library enhancements. The sixth edition was adopted by the General
Assembly of June 2015.
ECMAScript 2016 was the first ECMAScript edition released under Ecma TC39's new yearly release cadence and open
development process. A plain-text source document was built from the ECMAScript 2015 source document to serve as
the base for further development entirely on GitHub. Over the year of this standard's development, hundreds of
pull requests and issues were filed representing thousands of bug fixes, editorial fixes and other improvements.
Additionally, numerous software tools were developed to aid in this effort including Ecmarkup, Ecmarkdown, and
Grammarkdown. ES2016 also included support for a new exponentiation operator and adds a new method to
Array.prototype called includes.
ECMAScript 2017 introduced Async Functions, Shared Memory, and Atomics along with smaller language and library
enhancements, bug fixes, and editorial updates. Async functions improve the asynchronous programming experience
by providing syntax for promise-returning functions. Shared Memory and Atomics introduce a new memory model that allows
multi-agent programs to communicate using
atomic operations that ensure a well-defined execution order even on parallel CPUs. It also included new static
methods on Object: Object.values, Object.entries, and
Object.getOwnPropertyDescriptors.
ECMAScript 2018 introduced support for asynchronous iteration via the async
iterator protocol and async generators. It also included four new regular expression
features: the dotAll flag, named capture groups, Unicode property escapes, and look-behind
assertions. Lastly it included object rest and spread properties.
ECMAScript 2019 introduced a few new built-in functions: flat and flatMap on
Array.prototype for flattening arrays, Object.fromEntries for directly turning the
return value of Object.entries into a new Object, and trimStart and
trimEnd on String.prototype as better-named alternatives to the widely implemented but
non-standard String.prototype.trimLeft and trimRight built-ins. In addition, it
included a few minor updates to syntax and semantics. Updated syntax included optional catch binding parameters
and allowing U+2028 (LINE SEPARATOR) and U+2029 (PARAGRAPH SEPARATOR) in string literals to align with JSON.
Other updates included requiring that Array.prototype.sort be a stable sort, requiring that
JSON.stringify return well-formed UTF-8 regardless of input, and clarifying
Function.prototype.toString by requiring that it either return the corresponding original source
text or a standard placeholder.
ECMAScript 2020, the 11th edition, introduced the matchAll method for Strings, to
produce an iterator for all match objects generated by a global regular
expression; import(), a syntax to asynchronously import Modules with a dynamic specifier;
BigInt, a new number primitive for working with arbitrary precision integers; Promise.allSettled, a new Promise
combinator that does not short-circuit; globalThis, a universal way to access the global
this value; dedicated export * as ns from 'module' syntax for use within modules;
increased standardization of for-in enumeration order; import.meta, a host-populated object available in Modules that may
contain contextual information about the Module; as well as adding two new syntax features to improve working
with “nullish” values (undefined or null): nullish coalescing, a value
selection operator; and optional chaining, a property access and function invocation operator that
short-circuits if the value to access/invoke is nullish.
ECMAScript 2021, the 12th edition, introduced the replaceAll method for Strings;
Promise.any, a Promise combinator that short-circuits when an input value is fulfilled;
AggregateError, a new Error type to represent multiple errors at once; logical assignment operators
(??=, &&=, ||=); WeakRef, for referring to a target
object without preserving it from garbage collection, and FinalizationRegistry, to manage
registration and unregistration of cleanup operations performed when target objects are garbage collected;
separators for numeric literals (1_000); and Array.prototype.sort was made more
precise, reducing the amount of cases that result in an implementation-definedsort order.
ECMAScript 2022, the 13th edition, introduced top-level await, allowing the keyword to be used at the top level of modules; new
class elements: public and private instance fields, public and private static fields, private instance methods
and accessors, and private static methods and accessors; static blocks inside classes, to perform per-class
evaluation initialization; the #x in obj syntax, to test for presence of private fields on objects;
regular expression match indices via the /d flag, which provides start and end indices for matched
substrings; the cause property on Error objects, which can be used to record a
causation chain in errors; the at method for Strings, Arrays, and TypedArrays, which allows relative indexing; and
Object.hasOwn, a convenient alternative to Object.prototype.hasOwnProperty.
ECMAScript 2023, the 14th edition, introduced the toSorted, toReversed,
with, findLast, and findLastIndex methods on Array.prototype
and TypedArray.prototype, as well as the toSpliced method on
Array.prototype; added support for #! comments at the beginning of files to better
facilitate executable ECMAScript files; and allowed the use of most Symbols as keys in weak collections.
ECMAScript 2024, the 15th edition, added facilities for resizing and transferring ArrayBuffers and
SharedArrayBuffers; added a new RegExp /v flag for creating RegExps with more advanced features for
working with sets of strings; and introduced the Promise.withResolvers convenience method for
constructing Promises, the Object.groupBy and Map.groupBy methods for aggregating
data, the Atomics.waitAsync method for asynchronously waiting for a change to shared memory, and
the String.prototype.isWellFormed and String.prototype.toWellFormed methods for
checking and ensuring that strings contain only well-formed Unicode.
ECMAScript 2025, the 16th edition, added a new Iterator global with associated static
and prototype methods for working with iterators; added methods to Set.prototype for
performing common operations on Sets; added support for importing JSON modules as well as syntax for declaring
attributes of imported modules; added the RegExp.escape method for escaping a string to be safely
used in a regular expression; added syntax for enabling and disabling modifier flags inline within regular
expressions; added the Promise.try method for calling functions which may or may not return a
Promise and ensuring the result is always a Promise; and added a new
Float16ArrayTypedArray kind as well as the related
DataView.prototype.getFloat16, DataView.prototype.setFloat16, and
Math.f16round methods.
Dozens of individuals representing many organizations have made very significant contributions within Ecma TC39
to the development of this edition and to the prior editions. In addition, a vibrant community has emerged
supporting TC39's ECMAScript efforts. This community has reviewed numerous drafts, filed thousands of bug
reports, performed implementation experiments, contributed test suites, and educated the world-wide developer
community about ECMAScript. Unfortunately, it is impossible to identify and acknowledge every person and
organization who has contributed to this effort.
Allen Wirfs-Brock
ECMA-262, Project Editor, 6th Edition
Brian Terlson
ECMA-262, Project Editor, 7th through 10th Editions
Jordan Harband
ECMA-262, Project Editor, 10th through 12th Editions
Shu-yu Guo
ECMA-262, Project Editor, 12th through 16th Editions
Michael Ficarra
ECMA-262, Project Editor, 12th through 16th Editions
Kevin Gibbons
ECMA-262, Project Editor, 12th through 16th Editions
1 Scope
This Standard defines the ECMAScript 2026 general-purpose programming language.
2 Conformance
A conforming implementation of ECMAScript must provide and support all the types, values, objects, properties,
functions, and program syntax and semantics described in this specification.
A conforming implementation of ECMAScript must interpret source text input in conformance with the latest
version of the Unicode Standard and ISO/IEC 10646.
A conforming implementation of ECMAScript that provides an application programming interface (API) that
supports programs that need to adapt to the linguistic and cultural conventions used by different human
languages and countries must implement the interface defined by the most recent edition of ECMA-402 that is
compatible with this specification.
A conforming implementation of ECMAScript may provide additional types, values, objects, properties, and
functions beyond those described in this specification. In particular, a conforming implementation of ECMAScript
may provide properties not described in this specification, and values for those properties, for objects that
are described in this specification.
A conforming implementation of ECMAScript may support program and regular expression syntax not described in
this specification. In particular, a conforming implementation of ECMAScript may support program syntax that
makes use of any “future reserved words” noted in subclause 12.7.2 of this specification.
A conforming implementation of ECMAScript must not implement any extension that is listed as a Forbidden
Extension in subclause 17.1.
A conforming implementation of ECMAScript may choose to implement or not implement Normative
Optional subclauses. If any Normative Optional behaviour is implemented, all of the behaviour in the
containing Normative Optional clause must be implemented. A Normative Optional clause is denoted in this
specification with the words "Normative Optional" in a coloured box, as shown below.
A conforming implementation of ECMAScript must implement Legacy subclauses, unless
they are also marked as Normative Optional. All of the language features and behaviours specified within Legacy
subclauses have one or more undesirable characteristics. However, their continued usage in existing applications
prevents their removal from this specification. These features are not considered part of the core ECMAScript
language. Programmers should not use or assume the existence of these features and behaviours when writing new
ECMAScript code.
2.3 Example Legacy Normative Optional Clause Heading
Example clause contents.
3 Normative References
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
IEEE 754-2019,
IEEE Standard for Floating-Point Arithmetic.
ISO/IEC 10646, Information Technology — Universal Multiple-Octet Coded Character Set (UCS) plus
Amendment 1:2005, Amendment 2:2006, Amendment 3:2008, Amendment 4:2008, and additional amendments and
corrigenda, or successor.
This section contains a non-normative overview of the ECMAScript language.
ECMAScript is an object-oriented programming language for performing computations and manipulating
computational objects within a host environment. ECMAScript as defined here is not intended to be
computationally self-sufficient; indeed, there are no provisions in this specification for input of external
data or output of computed results. Instead, it is expected that the computational environment of an ECMAScript
program will provide not only the objects and other facilities described in this specification but also certain
environment-specific objects, whose description and behaviour are beyond the scope of this specification except
to indicate that they may provide certain properties that can be accessed and certain functions that can be
called from an ECMAScript program.
ECMAScript was originally designed to be used as a scripting language, but has become widely used as a
general-purpose programming language. A scripting language is a programming language that is used to
manipulate, customize, and automate the facilities of an existing system. In such systems, useful functionality
is already available through a user interface, and the scripting language is a mechanism for exposing that
functionality to program control. In this way, the existing system is said to provide a host environment of objects
and facilities, which completes the capabilities of the scripting language. A scripting language is intended for
use by both professional and non-professional programmers.
ECMAScript was originally designed to be a Web scripting language, providing a mechanism to enliven
Web pages in browsers and to perform server computation as part of a Web-based client-server architecture.
ECMAScript is now used to provide core scripting capabilities for a variety of host environments.
Therefore the core language is specified in this document apart from any particular host environment.
ECMAScript usage has moved beyond simple scripting and it is now used for the full spectrum of programming
tasks in many different environments and scales. As the usage of ECMAScript has expanded, so have the features
and facilities it provides. ECMAScript is now a fully featured general-purpose programming language.
4.1 Web Scripting
A web browser provides an ECMAScript host environment for client-side computation including, for
instance, objects that represent windows, menus, pop-ups, dialog boxes, text areas, anchors, frames, history,
cookies, and input/output. Further, the host environment provides a means to attach scripting code to
events such as change of focus, page and image loading, unloading, error and abort, selection, form
submission, and mouse actions. Scripting code appears within the HTML and the displayed page is a combination
of user interface elements and fixed and computed text and images. The scripting code is reactive to user
interaction, and there is no need for a main program.
A web server provides a different host environment for server-side computation including objects
representing requests, clients, and files; and mechanisms to lock and share data. By using browser-side and
server-side scripting together, it is possible to distribute computation between the client and server while
providing a customized user interface for a Web-based application.
Each Web browser and server that supports ECMAScript supplies its own host environment, completing the ECMAScript
execution environment.
4.2 Hosts and Implementations
To aid integrating ECMAScript into host environments, this specification defers the definition of
certain facilities (e.g., abstract operations), either in whole
or in part, to a source outside of this specification. Editorially, this specification distinguishes the
following kinds of deferrals.
An implementation is an external source that further defines facilities
enumerated in Annex D or those that are marked as implementation-defined or implementation-approximated. In informal use, an
implementation refers to a concrete artefact, such as a particular web browser.
An implementation-defined facility is one that defers
its definition to an external source without further qualification. This specification does not make any
recommendations for particular behaviours, and conforming implementations are free to choose any behaviour
within the constraints put forth by this specification.
An implementation-approximated facility is one that
defers its definition to an external source while recommending an ideal behaviour. While conforming
implementations are free to choose any behaviour within the constraints put forth by this specification, they
are encouraged to strive to approximate the ideal. Some mathematical operations, such as Math.exp, are implementation-approximated.
A host is an external source that further defines
facilities listed in Annex D but does not further define other implementation-defined or implementation-approximated facilities. In informal
use, a host refers to the set of all
implementations, such as the set of all web browsers, that interface with this specification in the same way
via Annex D. A host is often an external specification, such as WHATWG HTML (https://html.spec.whatwg.org/). In other words, facilities that are
host-defined are often
further defined in external specifications.
A host hook is an abstract operation that is
defined in whole or in part by an external source. All host hooks must be listed in Annex D. A
host hook must conform to at
least the following requirements:
A host-defined facility is one that defers its definition to an
external source without further qualification and is listed in Annex D.
Implementations that are not hosts may
also provide definitions for host-defined facilities.
A host environment is a
particular choice of definition for all host-defined facilities. A host environment typically includes objects or
functions which allow obtaining input and providing output as host-defined properties of the global object.
This specification follows the editorial convention of always using the most specific term. For example, if a
facility is host-defined,
it should not be referred to as implementation-defined.
Both hosts and implementations may
interface with this specification via the language types, specification types, abstract operations, grammar
productions, intrinsic objects, and intrinsic symbols defined herein.
4.3 ECMAScript Overview
The following is an informal overview of ECMAScript—not all parts of the language are described. This
overview is not part of the standard proper.
ECMAScript is object-based: basic language and host facilities are provided by objects, and an ECMAScript program is a
cluster of communicating objects. In ECMAScript, an object is a collection of zero or more
properties each with attributes that determine how each property can be used—for example,
when the Writable attribute for a property is set to false, any attempt by executed
ECMAScript code to assign a different value to the property fails. Properties are containers that hold other
objects, primitive values, or functions. A primitive value is a member of one of the
following built-in types: Undefined, Null, Boolean, Number, BigInt,
String, and Symbol; an object is a member of the built-in type Object; and a function is
a callable object. A function that is associated with an object via a property is called a method.
ECMAScript defines a collection of built-in objects that round out the definition of ECMAScript
entities. These built-in objects include the global object; objects that are fundamental to the runtime
semantics of the language including Object, Function,
Boolean, Symbol, and various Error objects; objects that represent and
manipulate numeric values including Math, Number, and Date; the text
processing objects String and RegExp; objects that are indexed collections of values
including Array and nine different kinds of Typed Arrays whose elements all have a specific
numeric data representation; keyed collections including Map and Set objects;
objects supporting structured data including the JSON object, ArrayBuffer,
SharedArrayBuffer, and DataView; objects supporting control abstractions including
generator functions and Promise objects; and reflection objects including Proxy and
Reflect.
ECMAScript also defines a set of built-in operators. ECMAScript operators include various unary
operations, multiplicative operators, additive operators, bitwise shift operators, relational operators,
equality operators, binary bitwise operators, binary logical operators, assignment operators, and the comma
operator.
Large ECMAScript programs are supported by modules which allow a program to be divided into multiple
sequences of statements and declarations. Each module explicitly identifies declarations it uses that need to
be provided by other modules and which of its declarations are available for use by other modules.
ECMAScript syntax intentionally resembles Java syntax. ECMAScript syntax is relaxed to enable it to serve as
an easy-to-use scripting language. For example, a variable is not required to have its type declared nor are
types associated with properties, and defined functions are not required to have their declarations appear
textually before calls to them.
4.3.1 Objects
Even though ECMAScript includes syntax for class definitions, ECMAScript objects are not fundamentally
class-based such as those in C++, Smalltalk, or Java. Instead objects may be created in various ways
including via a literal notation or via constructors which create objects and then execute code that
initializes all or part of them by assigning initial values to their properties. Each constructor is a function that
has a property named "prototype" that is used to implement prototype-based
inheritance and shared properties. Objects are created by using constructors in new
expressions; for example, new Date(2009, 11) creates a new Date object. Invoking a constructor without using
new has consequences that depend on the constructor. For example, Date() produces a string
representation of the current date and time rather than an object.
Every object created by a constructor has an implicit reference (called the object's
prototype) to the value of its constructor's "prototype" property. Furthermore, a
prototype may have a non-null implicit reference to its prototype, and so on; this is
called the prototype chain. When a reference is made to a property in an object, that reference is
to the property of that name in the first object in the prototype chain that contains a property of that
name. In other words, first the object mentioned directly is examined for such a property; if that object
contains the named property, that is the property to which the reference refers; if that object does not
contain the named property, the prototype for that object is examined next; and so on.
Figure 1: Object/Prototype Relationships
In a class-based object-oriented language, in general, state is carried by instances, methods are carried
by classes, and inheritance is only of structure and behaviour. In ECMAScript, the state and methods are
carried by objects, while structure, behaviour, and state are all inherited.
All objects that do not directly contain a particular property that their prototype contains share that
property and its value. Figure 1 illustrates this:
CF is a constructor (and also an object). Five objects have been created by
using new expressions: cf1, cf2, cf3,
cf4, and cf5. Each of these objects contains properties named
"q1" and "q2". The dashed lines represent the implicit prototype
relationship; so, for example, cf3's prototype is CFp. The constructor, CF, has two
properties itself, named "P1" and "P2", which are not visible to
CFp, cf1, cf2, cf3,
cf4, or cf5. The property named "CFP1" in
CFp is shared by cf1, cf2, cf3,
cf4, and cf5 (but not by CF), as are any properties found in
CFp's implicit prototype chain that are not named "q1",
"q2", or "CFP1". Notice that there is no implicit prototype link
between CF and CFp.
Unlike most class-based object languages, properties can be added to objects dynamically by assigning
values to them. That is, constructors are not required to name or assign values to all or any
of the constructed object's properties. In the above diagram, one could add a new shared property for
cf1, cf2, cf3, cf4, and
cf5 by assigning a new value to the property in CFp.
Although ECMAScript objects are not inherently class-based, it is often convenient to define class-like
abstractions based upon a common pattern of constructor functions, prototype objects, and methods. The ECMAScript
built-in objects themselves follow such a class-like pattern. Beginning with ECMAScript 2015, the ECMAScript
language includes syntactic class definitions that permit programmers to concisely define objects that
conform to the same class-like abstraction pattern used by the built-in objects.
4.3.2 The Strict Variant of ECMAScript
The ECMAScript Language recognizes the possibility that some users of the language may wish to restrict
their usage of some features available in the language. They might do so in the interests of security, to
avoid what they consider to be error-prone features, to get enhanced error checking, or for other reasons of
their choosing. In support of this possibility, ECMAScript defines a strict variant of the language. The
strict variant of the language excludes some specific syntactic and semantic features of the regular
ECMAScript language and modifies the detailed semantics of some features. The strict variant also specifies
additional error conditions that must be reported by throwing error exceptions in situations that are not
specified as errors by the non-strict form of the language.
The strict variant of ECMAScript is commonly referred to as the strict mode of the language.
Strict mode selection and use of the strict mode syntax and semantics of ECMAScript is explicitly made at
the level of individual ECMAScript source text units as described in 11.2.2. Because
strict mode is selected at the level of a syntactic source text unit, strict mode only imposes restrictions
that have local effect within such a source text unit. Strict mode does not restrict or modify any aspect of
the ECMAScript semantics that must operate consistently across multiple source text units. A complete
ECMAScript program may be composed of both strict mode and non-strict mode ECMAScript source text units. In this case,
strict mode only applies when actually executing code that is defined within a strict mode source text unit.
In order to conform to this specification, an ECMAScript implementation must implement both the full
unrestricted ECMAScript language and the strict variant of the ECMAScript language as defined by this
specification. In addition, an implementation must support the combination of unrestricted and strict mode
source text units into a single composite program.
4.4 Terms and Definitions
For the purposes of this document, the following terms and definitions apply.
4.4.1 implementation-approximated
an implementation-approximated facility is defined in
whole or in part by an external source but has a recommended, ideal behaviour in this specification
4.4.2 implementation-defined
an implementation-defined facility is defined in whole or in
part by an external source to this specification
The value of a constructor's "prototype" property is a
prototype object that is used to implement inheritance and shared properties.
4.4.8 prototype
object that provides shared properties for other objects
Note
When a constructor
creates an object, that object implicitly references the constructor's "prototype" property for the
purpose of resolving property references. The constructor's "prototype" property can be
referenced by the program expression constructor.prototype, and properties added
to an object's prototype are shared, through inheritance, by all objects sharing the prototype.
Alternatively, a new object may be created with an explicitly specified prototype by using the
Object.create built-in function.
4.4.9 ordinary object
object that has the default behaviour for the essential internal methods that must be supported by all
objects
4.4.10 exotic object
object that does not have the default behaviour for one or more of the essential internal methods
There are only two Boolean values, true and false.
4.4.18 Boolean type
type consisting of the primitive values true and false
4.4.19 Boolean object
member of the Object
type that is an instance of the standard built-in Boolean constructor
Note
A Boolean object is created by using the Boolean constructor in a new expression, supplying a Boolean
value as an argument. The resulting object has an internal slot whose value is the Boolean value. A
Boolean object can be coerced to a Boolean value.
4.4.20 String value
primitive value that is a finite
ordered sequence of zero or more 16-bit unsigned integer values
Note
A String value is a member of the String type. Each
integer value in the sequence
usually represents a single 16-bit unit of UTF-16 text. However, ECMAScript does not place any
restrictions or requirements on the values except that they must be 16-bit unsigned integers.
4.4.21 String type
set of all possible String values
4.4.22 String object
member of the Object
type that is an instance of the standard built-in String constructor
Note
A String object is created by using the String constructor in a new expression, supplying a String
value as an argument. The resulting object has an internal slot whose value is the String value. A
String object can be coerced to a String value by calling the String constructor as a function (22.1.1.1).
4.4.23 Number value
primitive value corresponding to a double-precision 64-bit binary format IEEE 754-2019 value
Note
A Number value is a member of the Number type and is
a direct representation of a number.
4.4.24 Number type
set of all possible Number values including NaN (“not a number”),
+∞𝔽 (positive infinity), and -∞𝔽 (negative
infinity)
4.4.25 Number object
member of the Object
type that is an instance of the standard built-in Number constructor
Note
A Number object is created by using the Number constructor in a new expression, supplying a Number
value as an argument. The resulting object has an internal slot whose value is the Number value. A
Number object can be coerced to a Number value by calling the Number constructor as a function (21.1.1.1).
4.4.26 Infinity
Number value that is the positive infinite Number value
4.4.27 NaN
Number value that is an IEEE
754-2019 NaN (“not a number”) value
4.4.28 BigInt value
primitive value corresponding to an arbitrary-precision integer value
4.4.29 BigInt type
set of all possible BigInt values
4.4.30 BigInt object
member of the Object
type that is an instance of the standard built-in BigInt constructor
4.4.31 Symbol value
primitive value that represents a unique, non-String Object property key
4.4.32 Symbol type
set of all possible Symbol values
4.4.33 Symbol object
member of the Object
type that is an instance of the standard built-in Symbol constructor
4.4.34 function
member of the Object
type that may be invoked as a subroutine
Note
In addition to its properties, a function contains executable code and state that determine how it
behaves when invoked. A function's code may or may not be written in ECMAScript.
4.4.35 built-in function
built-in object that is a function
Note
Examples of built-in functions include parseInt and Math.exp. A host or implementation may provide
additional built-in functions that are not described in this specification.
Examples of built-in constructors include Object and
Function. A host or
implementation may provide additional built-in constructors that are not described in this specification.
4.4.37 property
part of an object that associates a key (either a String value or a Symbol value) and a value
Note
Depending upon the form of the property the value may be represented either directly as a data value (a
primitive value, an object, or a function object) or indirectly by a pair of accessor
functions.
4.4.38 method
function that is the value of a property
Note
When a function is called as a method of an object, the object is passed to the function as its
this value.
4.4.39 built-in method
method that is a built-in function
Note
Standard built-in methods are defined in this specification. A host or implementation may provide additional built-in methods that are
not described in this specification.
4.4.40 attribute
internal value that defines some characteristic of a property
4.4.41 own property
property that is directly contained by its object
4.4.42 inherited property
property of an object that is not an own property but is a property (either own or inherited) of the
object's prototype
4.5 Organization of This Specification
The remainder of this specification is organized as follows:
Clause 5 defines the notational conventions used throughout the
specification.
Clauses 6 through 10 define the execution environment
within which ECMAScript programs operate.
Clauses 11 through 17 define the actual ECMAScript
programming language including its syntactic encoding and the execution semantics of all language features.
Clauses 18 through 28 define the ECMAScript
standard library. They include the definitions of all of the standard objects that are available for use by
ECMAScript programs as they execute.
Clause 29
describes the memory consistency model of accesses on SharedArrayBuffer-backed memory and methods of the
Atomics object.
5 Notational Conventions
5.1 Syntactic and Lexical Grammars
5.1.1 Context-Free Grammars
A context-free grammar consists of a number of productions. Each production has an
abstract symbol called a nonterminal as its left-hand side, and a sequence of zero or more
nonterminal and terminal symbols as its right-hand side. For each grammar, the terminal
symbols are drawn from a specified alphabet.
A chain production is a production that has exactly
one nonterminal symbol on its right-hand side along with zero or more terminal symbols.
Starting from a sentence consisting of a single distinguished nonterminal, called the goal symbol, a given context-free grammar specifies a
language, namely, the (perhaps infinite) set of possible sequences of terminal symbols that can
result from repeatedly replacing any nonterminal in the sequence with a right-hand side of a production for
which the nonterminal is the left-hand side.
Input elements other than white space and comments form the terminal symbols for the syntactic grammar for
ECMAScript and are called ECMAScript tokens. These tokens are the reserved
words, identifiers, literals, and punctuators of the ECMAScript language. Moreover, line
terminators, although not considered to be tokens, also become part of the stream of input elements and
guide the process of automatic semicolon insertion (12.10). Simple white space and
single-line comments are discarded and do not appear in the stream of input elements for the syntactic
grammar. A MultiLineComment (that is,
a comment of the form /*…*/ regardless of whether it spans more than one line) is
likewise simply discarded if it contains no line terminator; but if a MultiLineComment contains one or more line terminators, then
it is replaced by a single line terminator, which becomes part of the stream of input elements for the
syntactic grammar.
A RegExp grammar for ECMAScript is given in 22.2.1. This grammar also has as its terminal symbols the code
points as defined by SourceCharacter.
It defines a set of productions, starting from the goal symbolPattern, that describe how sequences of code points are translated
into regular expression patterns.
Productions of the lexical and RegExp grammars are distinguished by having two colons “::” as
separating punctuation. The lexical and RegExp grammars share some productions.
Productions of the numeric string grammar are distinguished by having three colons “:::” as
punctuation, and are never used for parsing source text.
5.1.4 The Syntactic Grammar
The syntactic grammar for ECMAScript is given in clauses 13 through 16. This grammar has ECMAScript
tokens defined by the lexical grammar as its terminal symbols (5.1.2). It defines a set of productions, starting
from two alternative goal symbolsScript and Module, that describe how sequences of tokens form syntactically
correct independent components of ECMAScript programs.
When a stream of code points is to be parsed as an ECMAScript Script or Module, it is first converted to a stream of input elements by repeated
application of the lexical grammar; this stream of input elements is then parsed by a single application of
the syntactic grammar. The input stream is syntactically in error if the tokens in the stream of input
elements cannot be parsed as a single instance of the goal nonterminal (Script or Module), with no tokens left over.
When a parse is successful, it constructs a parse tree, a rooted tree structure in which each node
is a Parse Node. Each Parse Node is an instance of
a symbol in the grammar; it represents a span of the source text that can be derived from that symbol. The
root node of the parse tree, representing the whole of the source text, is an instance of the parse's
goal
symbol. When a Parse Node is an instance of a nonterminal, it is also an instance of some
production that has that nonterminal as its left-hand side. Moreover, it has zero or more children,
one for each symbol on the production's right-hand side: each child is a Parse Node that is an instance of
the corresponding symbol.
New Parse Nodes are instantiated for each invocation of the parser and never reused between parses even of
identical source text. Parse Nodes are considered the same Parse Node if and only
if they represent the same span of source text, are instances of the same grammar symbol, and resulted from
the same parser invocation.
Note 1
Parsing the same String multiple times will lead to different Parse Nodes. For example, consider:
let str = "1 + 1;";
eval(str);
eval(str);
Each call to eval converts the value of str into ECMAScript source
text and performs an independent parse that creates its own separate tree of Parse
Nodes. The trees are distinct even though each parse operates upon a source text that was derived from
the same String value.
Note 2
Parse Nodes are specification artefacts, and implementations are not required to
use an analogous data structure.
Productions of the syntactic grammar are distinguished by having just one colon “:” as punctuation.
The syntactic grammar as presented in clauses 13 through 16 is not a complete account of which
token sequences are accepted as a correct ECMAScript Script or Module. Certain additional token sequences are also accepted, namely,
those that would be described by the grammar if only semicolons were added to the sequence in certain places
(such as before line terminator characters). Furthermore, certain token sequences that are described by the
grammar are not considered acceptable if a line terminator character appears in certain “awkward” places.
In certain cases, in order to avoid ambiguities, the syntactic grammar uses generalized productions that
permit token sequences that do not form a valid ECMAScript Script or Module. For example, this technique is used for object literals and
object destructuring patterns. In such cases a more restrictive supplemental grammar is provided
that further restricts the acceptable token sequences. Typically, an early error rule will then state that, in certain
contexts, "P must cover an N", where
P is a Parse Node (an instance of the generalized production) and N is a nonterminal
from the supplemental grammar. This means:
The sequence of tokens originally matched by P is parsed again using N as the
goal
symbol. If N takes grammatical parameters, then they are set to the same
values used when P was originally parsed.
If the sequence of tokens can be parsed as a single instance of N, with no tokens left over,
then:
We refer to that instance of N (a Parse Node, unique for a given P) as "the
N that is covered by P".
All Early Error rules for N and its derived productions also apply to the N
that is covered by P.
Otherwise (if the parse fails), it is an early Syntax Error.
5.1.5 Grammar Notation
5.1.5.1 Terminal Symbols
In the ECMAScript grammars, some terminal symbols are shown in fixed-width font. These are
to appear in a source text exactly as written. All terminal symbol code points specified in this way are
to be understood as the appropriate Unicode code points from the Basic Latin block, as opposed to any
similar-looking code points from other Unicode ranges. A code point in a terminal symbol cannot be
expressed by a \UnicodeEscapeSequence.
In grammars whose terminal symbols are individual Unicode code points (i.e., the lexical, RegExp, and
numeric string grammars), a contiguous run of multiple fixed-width code points appearing in a production
is a simple shorthand for the same sequence of code points, written as standalone terminal symbols.
In contrast, in the syntactic grammar, a contiguous run of fixed-width code points is a single terminal
symbol.
Terminal symbols come in two other forms:
In the lexical and RegExp grammars, Unicode code points without a conventional printed representation
are instead shown in the form "<ABBREV>" where "ABBREV" is a mnemonic for the code point or set of
code points. These forms are defined in Unicode Format-Control
Characters, White Space, and Line Terminators.
In the syntactic grammar, certain terminal symbols (e.g. IdentifierName and RegularExpressionLiteral) are shown in italics,
as they refer to the nonterminals of the same name in the lexical grammar.
5.1.5.2 Nonterminal Symbols and Productions
Nonterminal symbols are shown in italic type. The definition of a nonterminal (also called a
“production”) is introduced by the name of the nonterminal being defined followed by one or more colons.
(The number of colons indicates to which grammar the production belongs.) One or more alternative
right-hand sides for the nonterminal then follow on succeeding lines. For example, the syntactic
definition:
states that the nonterminal WhileStatement represents the token
while, followed by a left parenthesis token, followed by an Expression, followed by a right parenthesis token, followed by a
Statement. The occurrences of Expression and Statement are themselves nonterminals. As another example, the
syntactic definition:
states that an ArgumentList may represent either a single
AssignmentExpression or an
ArgumentList, followed
by a comma, followed by an AssignmentExpression. This definition of ArgumentList is recursive,
that is, it is defined in terms of itself. The result is that an ArgumentList may contain any positive number
of arguments, separated by commas, where each argument expression is an AssignmentExpression. Such recursive definitions of
nonterminals are common.
5.1.5.3 Optional Symbols
The subscripted suffix “opt”, which may appear after a terminal or nonterminal, indicates an
optional symbol. The alternative containing the optional symbol actually specifies two right-hand sides,
one that omits the optional element and one that includes it. This means that:
so, in this example, the nonterminal ForStatement actually has four alternative
right-hand sides.
5.1.5.4 Grammatical Parameters
A production may be parameterized by a subscripted annotation of the form “[parameters]”,
which may appear as a suffix to the nonterminal symbol defined by the production. “parameters”
may be either a single name or a comma separated list of names. A parameterized production is shorthand
for a set of productions defining all combinations of the parameter names, preceded by an underscore,
appended to the parameterized nonterminal symbol. This means that:
Prefixing a parameter name with “?” on a right-hand side nonterminal reference makes that
parameter value dependent upon the occurrence of the parameter name on the reference to the current
production's left-hand side symbol. For example:
If a right-hand side alternative is prefixed with “[+parameter]” that alternative is only available if
the named parameter was used in referencing the production's nonterminal symbol. If a right-hand side
alternative is prefixed with “[~parameter]” that alternative is only available if the named parameter was
not used in referencing the production's nonterminal symbol. This means that:
When the words “one of” follow the colon(s) in a grammar definition, they signify that each of the
terminal symbols on the following line or lines is an alternative definition. For example, the lexical
grammar for ECMAScript contains the production:
If the phrase “[empty]” appears as the right-hand side of a production, it indicates that the
production's right-hand side contains no terminals or nonterminals.
5.1.5.7 Lookahead Restrictions
If the phrase “[lookahead = seq]” appears in the right-hand side of a production, it indicates
that the production may only be used if the token sequence seq is a prefix of the immediately
following input token sequence. Similarly, “[lookahead ∈ set]”, where set is a
finite non-empty set of token
sequences, indicates that the production may only be used if some element of set is a prefix of
the immediately following token sequence. For convenience, the set can also be written as a nonterminal,
in which case it represents the set of all token sequences to which that nonterminal could expand. It is
considered an editorial error if the nonterminal could expand to infinitely many distinct token sequences.
These conditions may be negated. “[lookahead ≠ seq]” indicates that the containing production
may only be used if seq is not a prefix of the immediately following input token
sequence, and “[lookahead ∉ set]” indicates that the production may only be used if no
element of set is a prefix of the immediately following token sequence.
matches either the letter n followed by one or more decimal digits the first of which is
even, or a decimal digit not followed by another decimal digit.
Note that when these phrases are used in the syntactic grammar, it may not be possible to unambiguously
identify the immediately following token sequence because determining later tokens requires knowing which
lexical goal symbol to use at later positions. As such, when
these are used in the syntactic grammar, it is considered an editorial error for a token sequence
seq to appear in a lookahead restriction (including as part of a set of sequences) if the
choices of lexical goal symbols to use could change whether or not
seq would be a prefix of the resulting token sequence.
If the phrase “[no LineTerminator
here]” appears in the right-hand side of a production of the syntactic grammar, it indicates that the
production is a restricted production: it may not be used if a LineTerminator occurs in the input stream at the indicated
position. For example, the production:
indicates that the production may not be used if a LineTerminator occurs in the script between the
throw token and the Expression.
Unless the presence of a LineTerminator is forbidden by a restricted production, any
number of occurrences of LineTerminator may appear between any two consecutive tokens
in the stream of input elements without affecting the syntactic acceptability of the script.
5.1.5.9 but not
The right-hand side of a production may specify that certain expansions are not permitted by using the
phrase “but not” and then indicating the expansions to be excluded. For example, the production:
means that the nonterminal Identifier may be replaced by any sequence of
code points that could replace IdentifierName provided that the same sequence of code points
could not replace ReservedWord.
5.1.5.10 Descriptive Phrases
Finally, a few nonterminal symbols are described by a descriptive phrase in sans-serif type in cases
where it would be impractical to list all the alternatives:
The specification often uses a numbered list to specify steps in an algorithm. These algorithms are used to
precisely specify the required semantics of ECMAScript language constructs. The algorithms are not intended to
imply the use of any specific implementation technique. In practice, there may be more efficient algorithms
available to implement a given feature.
Algorithms may be explicitly parameterized with an ordered, comma-separated sequence of alias names which may
be used within the algorithm steps to reference the argument passed in that position. Optional parameters are
denoted with surrounding brackets ([ , name ]) and are no different from required parameters within
algorithm steps. A rest parameter may appear at the end of a parameter list, denoted with leading ellipsis (,
...name). The rest parameter captures all of the arguments provided following the required and
optional parameters into a List. If there are no such additional
arguments, that List is empty.
Algorithm steps may be subdivided into sequential substeps. Substeps are indented and may themselves be
further divided into indented substeps. Outline numbering conventions are used to identify substeps with the
first level of substeps labelled with lowercase alphabetic characters and the second level of substeps
labelled with lowercase roman numerals. If more than three levels are required these rules repeat with the
fourth level using numeric labels. For example:
Top-level step
Substep.
Substep.
Subsubstep.
Subsubsubstep
Subsubsubsubstep
Subsubsubsubsubstep
A step or substep may be written as an “if” predicate that conditions its substeps. In this case, the
substeps are only applied if the predicate is true. If a step or substep begins with the word “else”, it is a
predicate that is the negation of the preceding “if” predicate step at the same level.
A step may specify the iterative application of its substeps.
A step that begins with “Assert:” asserts an invariant condition of its
algorithm. Such assertions are used to make explicit algorithmic invariants that would otherwise be implicit.
Such assertions add no additional semantic requirements and hence need not be checked by an implementation.
They are used simply to clarify algorithms.
Algorithm steps may declare named aliases for any value using the form “Let x be
someValue”. These aliases are reference-like in that both x and someValue
refer to the same underlying data and modifications to either are visible to both. Algorithm steps that want
to avoid this reference-like behaviour should explicitly make a copy of the right-hand side: “Let x
be a copy of someValue” creates a shallow copy of someValue.
Once declared, an alias may be referenced in any subsequent steps and must not be referenced from steps prior
to the alias's declaration. Aliases may be modified using the form “Set x to
someOtherValue”.
5.2.1 Abstract Operations
In order to facilitate their use in multiple parts of this specification, some algorithms, called abstract operations, are named and written in parameterized functional form so that
they may be referenced by name from within other algorithms. Abstract operations are typically referenced
using a functional application style such as OperationName(arg1, arg2). Some abstract
operations are treated as polymorphically dispatched methods of class-like specification abstractions. Such
method-like abstract operations are typically referenced using a method application style such as
someValue.OperationName(arg1, arg2).
5.2.2 Syntax-Directed Operations
A syntax-directed operation is a named
operation whose definition consists of algorithms, each of which is associated with one or more productions
from one of the ECMAScript grammars. A production that has multiple alternative definitions will typically
have a distinct algorithm for each alternative. When an algorithm is associated with a grammar production,
it may reference the terminal and nonterminal symbols of the production alternative as if they were
parameters of the algorithm. When used in this manner, nonterminal symbols refer to the actual alternative
definition that is matched when parsing the source text. The source text matched by a grammar
production or Parse
Node derived from it is the portion of the source text that starts at the beginning of
the first terminal that participated in the match and ends at the end of the last terminal that participated
in the match.
When an algorithm is associated with a production alternative, the alternative is typically shown without
any “[ ]” grammar annotations. Such annotations should only affect the syntactic recognition of the
alternative and have no effect on the associated semantics for the alternative.
Syntax-directed operations are invoked with a parse node and, optionally, other parameters by using the
conventions on steps 1, 3, and 4 in the following algorithm:
Let status be SyntaxDirectedOperation of
SomeNonTerminal.
Let someParseNode be the parse of some source text.
Perform SyntaxDirectedOperation of someParseNode.
Perform SyntaxDirectedOperation of someParseNode with
argument "value".
Unless explicitly specified otherwise, all chain productions have an implicit definition for every
operation that might be applied to that production's left-hand side nonterminal. The implicit definition
simply reapplies the same operation with the same parameters, if any, to the chain
production's sole right-hand side nonterminal and then returns the result. For example,
assume that some algorithm has a step of the form: “Return Evaluation of Block” and that there is a production:
but the Evaluation
operation does not associate an algorithm with that production. In that case, the Evaluation operation implicitly
includes an association of the form:
The abstract operation Completion takes argument completionRecord (a Completion Record) and returns a
Completion Record. It is used to
emphasize that a Completion Record is being returned.
It performs the following steps when called:
Similarly, prefix ! is used to indicate that the following invocation of an abstract or
syntax-directed operation
will never return an abrupt completion and that the
resulting Completion Record's [[Value]] field should be used in place of the return value of the operation. For
example, the step:
In algorithms within abstract operations which are
declared to return a Completion Record, and within all
built-in functions, the returned value is first passed to NormalCompletion, and the result is used
instead. This rule does not apply within the Completion algorithm or when the value being returned is
clearly marked as a Completion Record in that step; these
cases are:
when the result of constructing a Completion Record is
directly returned
It is an editorial error if a Completion Record is
returned from such an abstract operation through any other means. For example, within these abstract operations,
Note that, through the ReturnIfAbrupt expansion, the following example is allowed,
as within the expanded steps, the result of applying Completion is returned directly in the abrupt case and the
implicit NormalCompletion application occurs after unwrapping in
the normal case.
Return ? completion.
The following example would be an editorial error because a Completion Record is being returned
without being annotated in that step.
Context-free grammars are not sufficiently powerful to express all the rules that define whether a stream
of input elements form a valid ECMAScript Script
or Module that may be evaluated. In some
situations additional rules are needed that may be expressed using either ECMAScript algorithm conventions
or prose requirements. Such rules are always associated with a production of a grammar and are called the
static semantics of the production.
Static Semantic Rules have names and typically are defined using an algorithm. Named Static Semantic Rules
are associated with grammar productions and a production that has multiple alternative definitions will
typically have for each alternative a distinct algorithm for each applicable named static semantic rule.
A special kind of static semantic rule is an Early Error
Rule. Early error
rules define early error
conditions (see clause 17) that are associated with specific
grammar productions. Evaluation of most early error rules are not explicitly invoked within the algorithms of
this specification. A conforming implementation must, prior to the first evaluation of a Script or Module, validate all of the early error rules of the productions used to parse
that Script or Module. If any of the early error rules are violated the Script or Module is invalid and cannot be evaluated.
5.2.5 Mathematical Operations
This specification makes reference to these kinds of numeric values:
Mathematical values: Arbitrary real numbers, used as the default numeric type.
In the language of this specification, numerical values are distinguished among different numeric kinds
using subscript suffixes. The subscript 𝔽 refers to Numbers, and the subscript ℤ
refers to BigInts. Numeric values without a subscript suffix refer to mathematical values. This specification
denotes most numeric values in base 10; it also uses numeric values of the form 0x followed by digits 0-9 or
A-F as base-16 values.
In general, when this specification refers to a numerical value, such as in the phrase, "the length of
y" or "the integer
represented by the four hexadecimal digits ...", without explicitly specifying a numeric kind, the phrase
refers to a mathematical
value. Phrases which refer to a Number or a BigInt value are explicitly annotated as
such; for example, "the Number
value for the number of code points in …" or "the BigInt value for …".
When the term integer is used in this specification, it refers to a mathematical value
which is in the set of integers,
unless otherwise stated. When the term integral Number is used in this specification, it refers to a
finite Number value whose mathematical value
is in the set of integers.
Numeric operators such as +, ×, =, and ≥ refer to those operations as determined by the type of the
operands. When applied to mathematical values, the operators refer to the usual
mathematical operations. When applied to extended mathematical values, the operators refer to
the usual mathematical operations over the extended real numbers; indeterminate forms are not defined and
their use in this specification should be considered an editorial error. When applied to Numbers, the
operators refer to the relevant operations within IEEE 754-2019. When applied to BigInts, the operators refer to
the usual mathematical operations applied to the mathematical value of the BigInt. Numeric operators applied
to mixed-type operands (such as a Number and a mathematical value) are not defined and should be considered
an editorial error in this specification.
The mathematical function abs(x)
produces the absolute value of x, which is -x if
x < 0 and otherwise is x itself.
The mathematical function min(x1, x2,
… , xN) produces the mathematically smallest of x1 through xN. The
mathematical function max(x1, x2, ...,
xN) produces the mathematically largest of x1 through xN. The domain
and range of these mathematical functions are the extended mathematical values.
The notation “x modulo
y” (y must be finite and non-zero) computes a value k of the same sign as
y (or zero) such that abs(k) < abs(y) and x - k = q ×
y for some integerq.
The phrase "the result of clamping x between
lower and upper" (where x is an extended
mathematical value and lower and upper are mathematical values
such that lower ≤ upper) produces lower if x <
lower, produces upper if x > upper, and otherwise produces
x.
The mathematical function floor(x)
produces the largest integer
(closest to +∞) that is not larger than x.
The mathematical function truncate(x) removes the fractional part of x by rounding
towards zero, producing -floor(-x) if x < 0 and otherwise
producing floor(x).
Mathematical functions min, max, abs, floor, and truncate are not defined for Numbers and BigInts, and any usage of
those methods that have non-mathematical value arguments would be an editorial error in
this specification.
An interval from lower bound a to upper bound
b is a possibly-infinite, possibly-empty set of numeric values of the same numeric type. Each
bound will be described as either inclusive or exclusive, but not both. There are four kinds of intervals,
as follows:
An interval from
a (inclusive) to b (inclusive), also called an inclusive interval from a to b, includes all values
x of the same numeric type such that a ≤ x ≤ b, and no others.
An interval from
a (inclusive) to b (exclusive) includes all values x of the same numeric
type such that a ≤ x < b, and no others.
An interval from
a (exclusive) to b (inclusive) includes all values x of the same numeric
type such that a < x ≤ b, and no others.
An interval from
a (exclusive) to b (exclusive) includes all values x of the same numeric
type such that a < x < b, and no others.
For example, the interval from
1 (inclusive) to 2 (exclusive) consists of all mathematical values between 1 and 2, including 1 and not
including 2. For the purpose of defining intervals, -0𝔽 <
+0𝔽, so, for example, an inclusive interval with a lower bound of
+0𝔽 includes +0𝔽 but not
-0𝔽. NaN is never included in an interval.
5.2.6 Value Notation
In this specification, ECMAScript language values are displayed in
bold. Examples include null, true, or
"hello". These are distinguished from ECMAScript source text such as
Function.prototype.apply or let n = 42;.
5.2.7 Identity
In this specification, both specification values and ECMAScript language values are
compared for equality. When comparing for equality, values fall into one of two categories. Values without identity are
equal to other values without identity if all of their innate characteristics are the same — characteristics
such as the magnitude of an integer or the length of a sequence. Values without identity may be
manifest without prior reference by fully describing their characteristics. In contrast, each value with identity is unique and therefore only equal
to itself. Values with identity are like values without identity but with an additional unguessable,
unchangeable, universally-unique characteristic called identity. References to existing values with
identity cannot be manifest simply by describing them, as the identity itself is indescribable; instead,
references to these values must be explicitly passed from one place to another. Some values with identity
are mutable and therefore can have their characteristics (except their identity) changed in-place, causing
all holders of the value to observe the new characteristics. A value without identity is never equal to a
value with identity.
From the perspective of this specification, the word “is” is used to compare two values for equality, as in
“If bool is true, then ...”, and the word “contains” is used to search for a
value inside lists using equality comparisons, as in "If list contains a Recordr such that
r.[[Foo]] is true, then ...". The specification
identity of values determines the result of these comparisons and is axiomatic in this specification.
From the perspective of the ECMAScript language, language values are compared for equality using the
SameValue abstract
operation and the abstract operations it transitively
calls. The algorithms of these comparison abstract operations
determine language identity of ECMAScript language values.
Algorithms within this specification manipulate values each of which has an associated type. The possible value
types are exactly those defined in this clause. Types are further classified into ECMAScript
language types and specification types.
6.1 ECMAScript Language Types
An ECMAScript language type corresponds to
values that are directly manipulated by an ECMAScript programmer using the ECMAScript language. The ECMAScript
language types are Undefined, Null, Boolean, String, Symbol, Number, BigInt, and Object. An ECMAScript language value is a value that is
characterized by an ECMAScript language type.
6.1.1 The Undefined Type
The Undefined type has exactly one value, called undefined. Any variable that has not
been assigned a value has the value undefined.
6.1.2 The Null Type
The Null type has exactly one value, called null.
6.1.3 The Boolean Type
The Boolean type represents a logical
entity having two values, called true and false.
6.1.4 The String Type
The String type is the set of all ordered
sequences of zero or more 16-bit unsigned integer values (“elements”) up to a maximum length of 253 - 1
elements. The String type is generally used to represent textual data in a running ECMAScript program, in
which case each element in the String is treated as a UTF-16 code unit value. Each element is regarded as
occupying a position within the sequence. These positions are indexed with non-negative integers. The first element (if any) is
at index 0, the next element (if any) at index 1, and so on. The length of a String is the number of
elements (i.e., 16-bit values) within it. The empty String has length zero and therefore contains no
elements.
ECMAScript operations that do not interpret String contents apply no further semantics. Operations that do
interpret String values treat each element as a single UTF-16 code unit. However, ECMAScript does not
restrict the value of or relationships between these code units, so operations that further interpret String
contents as sequences of Unicode code points encoded in UTF-16 must account for ill-formed subsequences.
Such operations apply special treatment to every code unit with a numeric value in the inclusive interval
from 0xD800 to 0xDBFF (defined by the Unicode Standard as a leading surrogate, or more formally as a high-surrogate code
unit) and every code unit with a numeric value in the inclusive interval from 0xDC00 to 0xDFFF
(defined as a trailing
surrogate, or more formally as a low-surrogate code unit) using the following
rules:
A sequence of two code units, where the first code unit c1 is a leading surrogate
and the second code unit c2 a trailing surrogate, is a surrogate pair and is interpreted as a code point with
the value (c1 - 0xD800) × 0x400 + (c2 - 0xDC00) + 0x10000. (See 11.1.3)
The function String.prototype.normalize (see 22.1.3.15) can be used to explicitly
normalize a String value. String.prototype.localeCompare (see 22.1.3.12) internally normalizes String
values, but no other operations implicitly normalize the strings upon which they operate. Operation results
are not language- and/or locale-sensitive unless stated otherwise.
Note
The rationale behind this design was to keep the implementation of Strings as simple and
high-performing as possible. If ECMAScript source text is in Normalized Form C, string
literals are guaranteed to also be normalized, as long as they do not contain any Unicode escape
sequences.
In this specification, the phrase "the string-concatenation of A, B, ..." (where each argument is a
String value, a code unit, or a sequence of code units) denotes the String value whose sequence of code
units is the concatenation of the code units (in order) of each of the arguments (in order).
The phrase "the substring of S from
inclusiveStart to exclusiveEnd" (where S is a String value or a sequence of
code units and inclusiveStart and exclusiveEnd are integers) denotes the String value consisting of the
consecutive code units of S beginning at index inclusiveStart and ending immediately
before index exclusiveEnd (which is the empty String when inclusiveStart =
exclusiveEnd). If the "to" suffix is omitted, the length of S is used as the value of
exclusiveEnd.
The phrase "the ASCII word characters" denotes the
following String value, which consists solely of every letter and number in the Unicode Basic Latin block
along with U+005F (LOW LINE): "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_".
For historical reasons, it has significance to various algorithms.
The abstract operation StringIndexOf takes arguments string (a String), searchValue
(a String), and fromIndex (a non-negative integer) and returns a non-negative integer or not-found. It
performs the following steps when called:
Let len be the length of string.
If searchValue is the empty String and fromIndex ≤ len, return
fromIndex.
Let searchLen be the length of searchValue.
For each integeri such that fromIndex ≤ i ≤ len - searchLen, in
ascending order, do
Let candidate be the substring of string from i to
i + searchLen.
If candidate is searchValue, return i.
Return not-found.
Note 1
If searchValue is the empty String and fromIndex ≤ the length of
string, this algorithm returns fromIndex. The empty String is effectively found
at every position within a string, including after the last code unit.
Note 2
This algorithm always returns not-found if fromIndex + the length
of searchValue > the length of string.
The abstract operation StringLastIndexOf takes arguments string (a String),
searchValue (a String), and fromIndex (a non-negative integer) and returns a non-negative integer or
not-found. It performs the following steps when called:
For each integeri such that 0 ≤ i ≤ fromIndex, in descending order, do
Let candidate be the substring of string from i to
i + searchLen.
If candidate is searchValue, return i.
Return not-found.
Note
If searchValue is the empty String, this algorithm returns fromIndex. The empty
String is effectively found at every position within a string, including after the last code unit.
6.1.5 The Symbol Type
The Symbol type is the set of all
non-String values that may be used as the key of an Object property (6.1.7).
Each Symbol is unique and immutable.
Each Symbol has an immutable [[Description]] internal slot whose value is either a
String or undefined.
6.1.5.1 Well-Known Symbols
Well-known symbols are built-in Symbol values that are explicitly referenced by algorithms of this
specification. They are typically used as the keys of properties whose values serve as extension points of
a specification algorithm. Unless otherwise specified, well-known symbols values are shared by all
realms (9.3).
Within this specification a well-known symbol is referred to using the standard intrinsic notation where the intrinsic is one
of the values listed in Table 1.
Note
Previous editions of this specification used a notation of the form @@name,
where the current edition would use %Symbol.name%. In particular, the following names were
used: @@asyncIterator, @@hasInstance, @@isConcatSpreadable, @@iterator,
@@match, @@matchAll, @@replace, @@search, @@species, @@split, @@toPrimitive, @@toStringTag, and
@@unscopables.
Table 1: Well-known Symbols
Specification Name
[[Description]]
Value and Purpose
%Symbol.asyncIterator%
"Symbol.asyncIterator"
A method that returns the default async iterator for an
object. Called by the semantics of the for-await-of
statement.
%Symbol.hasInstance%
"Symbol.hasInstance"
A method that determines if a constructor object recognizes an object as one of the
constructor's
instances. Called by the semantics of the instanceof operator.
%Symbol.isConcatSpreadable%
"Symbol.isConcatSpreadable"
A Boolean valued property that if true indicates that an object should be flattened to its array
elements by Array.prototype.concat.
%Symbol.iterator%
"Symbol.iterator"
A method that returns the default iterator for an object. Called by the
semantics of the for-of statement.
%Symbol.match%
"Symbol.match"
A regular expression method that matches the regular expression against a string. Called by the
String.prototype.match
method.
%Symbol.matchAll%
"Symbol.matchAll"
A regular expression method that returns an iterator that yields matches
of the regular expression against a string. Called by the String.prototype.matchAll
method.
%Symbol.replace%
"Symbol.replace"
A regular expression method that replaces matched substrings of a string. Called by the
String.prototype.replace
method.
%Symbol.search%
"Symbol.search"
A regular expression method that returns the index within a string that matches the regular
expression. Called by the String.prototype.search
method.
%Symbol.species%
"Symbol.species"
A function valued property that is the constructor function that is used to create derived
objects.
%Symbol.split%
"Symbol.split"
A regular expression method that splits a string at the indices that match the regular
expression. Called by the String.prototype.split
method.
%Symbol.toPrimitive%
"Symbol.toPrimitive"
A method that converts an object to a corresponding primitive value. Called by the ToPrimitive
abstract operation.
%Symbol.toStringTag%
"Symbol.toStringTag"
A String valued property that is used in the creation of the default string description of an
object. Accessed by the built-in method Object.prototype.toString.
%Symbol.unscopables%
"Symbol.unscopables"
An object valued property whose own and inherited property names are property names that are
excluded from the with environment bindings of the associated object.
6.1.6 Numeric Types
ECMAScript has two built-in numeric types: Number and BigInt. The following abstract operations are defined
over these numeric types. The "Result" column shows the return type, along with an indication if it is
possible for some invocations of the operation to return an abrupt completion.
Because the numeric types are in general not convertible without loss of precision or truncation, the
ECMAScript language provides no implicit conversion among these types. Programmers must explicitly call
Number and BigInt functions to convert among types when calling a function which
requires another type.
Note
The first and subsequent editions of ECMAScript have provided, for certain operators, implicit numeric
conversions that could lose precision or truncate. These legacy implicit conversions are maintained for
backward compatibility, but not provided for BigInt in order to minimize opportunity for programmer
error, and to leave open the option of generalized value types in a future edition.
6.1.6.1 The Number Type
The Number type has exactly
18,437,736,874,454,810,627 (that is, 264 - 253 +
3) values, representing the double-precision floating point IEEE 754-2019 binary64 values as specified in
the IEEE Standard for Binary Floating-Point Arithmetic, except that the 9,007,199,254,740,990 (that is,
253 - 2) distinct NaN values of the IEEE Standard are
represented in ECMAScript as a single special NaN value. (Note that the
NaN value is produced by the program expression NaN.) In some
implementations, external code might be able to detect a difference between various NaN values, but such
behaviour is implementation-defined; to ECMAScript code, all
NaN values are indistinguishable from each other.
Note
The bit pattern that might be observed in an ArrayBuffer (see 25.1)
or a SharedArrayBuffer (see 25.2) after a Number value has been stored
into it is not necessarily the same as the internal representation of that Number value used by the
ECMAScript implementation.
There are two other special values, called positive Infinity and negative
Infinity. For brevity, these values are also referred to for expository purposes by the
symbols +∞𝔽 and -∞𝔽, respectively. (Note
that these two infinite Number values are produced by the program expressions +Infinity (or
simply Infinity) and -Infinity.)
The other 18,437,736,874,454,810,624 (that is, 264 -
253) values are called the finite numbers.
Half of these are positive numbers and half are negative numbers; for every finite positive Number value there is a corresponding
negative value having the same magnitude.
Note that there is both a positive zero and a negative zero. For
brevity, these values are also referred to for expository purposes by the symbols
+0𝔽 and -0𝔽, respectively. (Note that these
two different zero Number values are produced by the program expressions +0 (or simply
0) and -0.)
The 18,437,736,874,454,810,622 (that is, 264 - 253 -
2) finite non-zero
values are of two kinds:
18,428,729,675,200,069,632 (that is, 264 - 254)
of them are normalized, having the form
The remaining 9,007,199,254,740,990 (that is, 253 - 2)
values are denormalized, having the form
s × m × 2e
where s is 1 or -1, m is an integer in the interval from 0 (exclusive) to 252 (exclusive), and
e is -1074.
Note that all the positive and negative integers whose magnitude is no greater than 253 are
representable in the Number type. The integer 0 has two representations in the Number type:
+0𝔽 and -0𝔽.
A finite number has an odd
significand if it is non-zero and the integerm used to express it (in one of the two forms shown
above) is odd. Otherwise, it has an even significand.
In this specification, the phrase “the Number value for x” where x
represents an exact real mathematical quantity (which might even be an irrational number such as π) means
a Number value chosen in the following manner. Consider the set of all finite values of the Number type, with
-0𝔽 removed and with two additional values added to it that are not
representable in the Number type, namely 21024 (which is +1 ×
253 × 2971) and -21024
(which is -1 × 253 × 2971). Choose the member of
this set that is closest in value to x. If two values of the set are equally close, then the
one with an even significand is chosen; for this purpose, the two extra values 21024 and
-21024 are considered to have even significands. Finally, if
21024 was chosen, replace it with +∞𝔽; if -21024 was chosen, replace it with
-∞𝔽; if +0𝔽 was chosen, replace it with
-0𝔽 if and only if x < 0; any other chosen value is used
unchanged. The result is the Number value forx. (This procedure corresponds
exactly to the behaviour of the IEEE 754-2019 roundTiesToEven mode.)
Some ECMAScript operators deal only with integers in specific ranges such as the inclusive
interval from -231 to 231 - 1 or the inclusive interval from 0 to 216 - 1. These operators accept any value of the Number type but
first convert each such value to an integer value in the expected range. See the descriptions of the
numeric conversion operations in 7.1.
6.1.6.1.1 Number::unaryMinus ( x )
The abstract operation Number::unaryMinus takes argument x (a Number) and returns a Number.
It performs the following steps when called:
If x is NaN, return NaN.
Return the negation of x; that is, compute a Number with the same magnitude but
opposite sign.
6.1.6.1.2 Number::bitwiseNOT ( x )
The abstract operation Number::bitwiseNOT takes argument x (a Number) and returns an
integral
Number. It performs the following steps when called:
Return the bitwise complement of oldValue. The mathematical value of the result is
exactly representable as a 32-bit two's complement bit string.
6.1.6.1.3 Number::exponentiate ( base, exponent )
The abstract operation Number::exponentiate takes arguments base (a Number) and
exponent (a Number) and returns a Number. It returns an implementation-approximated value representing
the result of raising base to the exponent power. It performs the following steps
when called:
If exponent is NaN, return NaN.
If exponent is either +0𝔽 or
-0𝔽, return 1𝔽.
If base is NaN, return NaN.
If base is +∞𝔽, then
If exponent > +0𝔽, return
+∞𝔽; otherwise return +0𝔽.
If base is -∞𝔽, then
If exponent > +0𝔽, then
If exponent is an odd integral Number, return
-∞𝔽; otherwise return +∞𝔽.
Else,
If exponent is an odd integral Number, return
-0𝔽; otherwise return +0𝔽.
If base is +0𝔽, then
If exponent > +0𝔽, return
+0𝔽; otherwise return +∞𝔽.
If base is -0𝔽, then
If exponent > +0𝔽, then
If exponent is an odd integral Number, return
-0𝔽; otherwise return +0𝔽.
Else,
If exponent is an odd integral Number, return
-∞𝔽; otherwise return +∞𝔽.
Assert: base
is finite and is neither
+0𝔽 nor -0𝔽.
Assert:
exponent is finite and is neither +0𝔽 nor
-0𝔽.
If base < -0𝔽 and exponent is not an
integral
Number, return NaN.
Return an implementation-approximated Number value
representing the result of raising ℝ(base) to the ℝ(exponent) power.
Note
The result of base**exponent when base is
1𝔽 or -1𝔽 and exponent is
+∞𝔽 or -∞𝔽, or when base is
1𝔽 and exponent is NaN, differs from
IEEE
754-2019. The first edition of ECMAScript specified a result of
NaN for this operation, whereas later revisions of IEEE 754 specified
1𝔽. The historical ECMAScript behaviour is preserved for
compatibility reasons.
6.1.6.1.4 Number::multiply ( x, y )
The abstract operation Number::multiply takes arguments x (a Number) and y (a
Number) and returns a Number. It performs multiplication according to the rules of IEEE 754-2019
binary double-precision arithmetic, producing the product of x and y. It performs
the following steps when called:
Finite-precision
multiplication is commutative, but not always associative.
6.1.6.1.5 Number::divide ( x, y )
The abstract operation Number::divide takes arguments x (a Number) and y (a
Number) and returns a Number. It performs division according to the rules of IEEE 754-2019
binary double-precision arithmetic, producing the quotient of x and y where
x is the dividend and y is the divisor. It performs the following steps when
called:
If x is NaN or y is NaN, return
NaN.
If x is either +∞𝔽 or
-∞𝔽, then
If y is either +∞𝔽 or
-∞𝔽, return NaN.
If y is +0𝔽 or y >
+0𝔽, return x.
Return -x.
If y is +∞𝔽, then
If x is +0𝔽 or x >
+0𝔽, return +0𝔽; otherwise return
-0𝔽.
If y is -∞𝔽, then
If x is +0𝔽 or x >
+0𝔽, return -0𝔽; otherwise return
+0𝔽.
The abstract operation Number::remainder takes arguments n (a Number) and d (a
Number) and returns a Number. It yields the remainder from an implied division of its operands where
n is the dividend and d is the divisor. It performs the following steps when
called:
In C and C++, the remainder operator accepts only integral operands; in ECMAScript, it also accepts
floating-point operands.
Note 2
The result of a floating-point remainder operation as computed by the
% operator is not the same as the “remainder” operation defined by IEEE 754-2019.
The IEEE
754-2019 “remainder” operation computes the remainder from a rounding division, not
a truncating division, and so its behaviour is not analogous to that of the usual
integer remainder operator. Instead the ECMAScript language defines
% on floating-point operations to behave in a manner analogous to that of the Java
integer remainder operator; this may be compared with the C library
function fmod.
6.1.6.1.7 Number::add ( x, y )
The abstract operation Number::add takes arguments x (a Number) and y (a Number)
and returns a Number. It performs addition according to the rules of IEEE 754-2019 binary double-precision
arithmetic, producing the sum of its arguments. It performs the following steps when called:
Finite-precision addition
is commutative, but not always associative.
6.1.6.1.8 Number::subtract ( x, y )
The abstract operation Number::subtract takes arguments x (a Number) and y (a
Number) and returns a Number. It performs subtraction, producing the difference of its operands;
x is the minuend and y is the subtrahend. It performs the following steps when
called:
It is always the case that x - y produces the same result as x + (-y).
6.1.6.1.9 Number::leftShift ( x, y )
The abstract operation Number::leftShift takes arguments x (a Number) and y (a
Number) and returns an integral Number. It performs the following steps when called:
Return the result of left shifting lNum by shiftCount bits. The mathematical value
of the result is exactly representable as a 32-bit two's complement bit string.
6.1.6.1.10 Number::signedRightShift ( x, y )
The abstract operation Number::signedRightShift takes arguments x (a Number) and
y (a Number) and returns an integral Number. It performs the following steps when called:
Return the result of performing a sign-extending right shift of lNum by
shiftCount bits. The most significant bit is propagated. The mathematical value
of the result is exactly representable as a 32-bit two's complement bit string.
6.1.6.1.11 Number::unsignedRightShift ( x, y )
The abstract operation Number::unsignedRightShift takes arguments x (a Number) and
y (a Number) and returns an integral Number. It performs the following steps when called:
Return the result of performing a zero-filling right shift of lNum by
shiftCount bits. Vacated bits are filled with zero. The mathematical value
of the result is exactly representable as a 32-bit unsigned bit string.
6.1.6.1.12 Number::lessThan ( x, y )
The abstract operation Number::lessThan takes arguments x (a Number) and y (a
Number) and returns a Boolean or undefined. It performs the following steps when
called:
If ℝ(x) < ℝ(y), return
true; otherwise return false.
6.1.6.1.13 Number::equal ( x, y )
The abstract operation Number::equal takes arguments x (a Number) and y (a
Number) and returns a Boolean. It performs the following steps when called:
If x is NaN, return false.
If y is NaN, return false.
If x is y, return true.
If x is +0𝔽 and y is
-0𝔽, return true.
If x is -0𝔽 and y is
+0𝔽, return true.
Return false.
6.1.6.1.14 Number::sameValue ( x, y )
The abstract operation Number::sameValue takes arguments x (a Number) and y (a
Number) and returns a Boolean. It performs the following steps when called:
If x is NaN and y is NaN, return
true.
If x is +0𝔽 and y is
-0𝔽, return false.
If x is -0𝔽 and y is
+0𝔽, return false.
If x is y, return true.
Return false.
6.1.6.1.15 Number::sameValueZero ( x, y )
The abstract operation Number::sameValueZero takes arguments x (a Number) and y
(a Number) and returns a Boolean. It performs the following steps when called:
If x is NaN and y is NaN, return
true.
If x is +0𝔽 and y is
-0𝔽, return true.
If x is -0𝔽 and y is
+0𝔽, return true.
If x is y, return true.
Return false.
6.1.6.1.16 NumberBitwiseOp ( op, x, y )
The abstract operation NumberBitwiseOp takes arguments op (&,
^, or |), x (a Number), and y (a Number) and returns an
integral
Number. It performs the following steps when called:
Let result be the result of applying the bitwise inclusive OR operation to
lBits and rBits.
Return the Number
value for the integer represented by the 32-bit two's complement bit string
result.
6.1.6.1.17 Number::bitwiseAND ( x, y )
The abstract operation Number::bitwiseAND takes arguments x (a Number) and y (a
Number) and returns an integral Number. It performs the following steps when called:
The abstract operation Number::bitwiseXOR takes arguments x (a Number) and y (a
Number) and returns an integral Number. It performs the following steps when called:
The abstract operation Number::bitwiseOR takes arguments x (a Number) and y (a
Number) and returns an integral Number. It performs the following steps when called:
The abstract operation Number::toString takes arguments x (a Number) and radix
(an integer in the inclusive
interval from 2 to 36) and returns a String. It represents x as a String
using a positional numeral system with radix radix. The digits used in the representation of
a number using radix r are taken from the first r code units of
"0123456789abcdefghijklmnopqrstuvwxyz" in order. The representation of numbers with
magnitude greater than or equal to 1𝔽 never includes leading zeroes. It
performs the following steps when called:
Let n, k, and s be
integers such that
k ≥ 1, radixk - 1 ≤ s <
radixk, 𝔽(s × radixn -
k) is x, and k is as small as possible. Note that
k is the number of digits in the representation of s using radix
radix, that s is not divisible by radix, and that the least
significant digit of s is not necessarily uniquely determined by these criteria.
the code unit of the most significant digit of the decimal representation of s
the code unit 0x002E (FULL STOP)
the code units of the remaining k - 1 digits of the decimal representation of
s
the code unit 0x0065 (LATIN SMALL LETTER E)
exponentSign
the code units of the decimal representation of abs(n - 1)
Note 1
The following observations may be useful as guidelines for implementations, but are not part of the
normative requirements of this Standard:
If x is any Number value other than -0𝔽, then ToNumber(ToString(x)) is x.
The least significant digit of s is not always uniquely determined by the requirements listed in
step 5.
Note 2
For implementations that provide more accurate conversions than required by the rules above, it is
recommended that the following alternative version of step 5 be used as a guideline:
Let n, k, and s be integers such that k ≥ 1,
radixk - 1 ≤ s <
radixk, 𝔽(s × radixn -
k) is x, and k is as small as possible. If there are
multiple possibilities for s, choose the value of s for which s
× radixn - k is closest in value to ℝ(x). If there are two such
possible values of s, choose the one that is even. Note that k is the
number of digits in the representation of s using radix radix and that
s is not divisible by radix.
Note 3
Implementers of ECMAScript may find useful the paper and code written by David M. Gay for
binary-to-decimal conversion of floating-point numbers:
The BigInt type represents an integer value. The value may be any
size and is not limited to a particular bit-width. Generally, where not otherwise noted, operations are
designed to return exact mathematically-based answers. For binary operations, BigInts act as two's
complement binary strings, with negative numbers treated as having bits set infinitely to the left.
6.1.6.2.1 BigInt::unaryMinus ( x )
The abstract operation BigInt::unaryMinus takes argument x (a BigInt) and returns a BigInt.
It performs the following steps when called:
If x = 0ℤ, return 0ℤ.
Return -x.
6.1.6.2.2 BigInt::bitwiseNOT ( x )
The abstract operation BigInt::bitwiseNOT takes argument x (a BigInt) and returns a BigInt.
It returns the one's complement of x. It performs the following steps when called:
Return -x - 1ℤ.
6.1.6.2.3 BigInt::exponentiate ( base, exponent )
The abstract operation BigInt::exponentiate takes arguments base (a BigInt) and
exponent (a BigInt) and returns either a normal completion containing a
BigInt or a throw completion. It performs the
following steps when called:
If exponent < 0ℤ, throw a
RangeError exception.
If base = 0ℤ and exponent =
0ℤ, return 1ℤ.
Return base raised to the power exponent.
6.1.6.2.4 BigInt::multiply ( x, y )
The abstract operation BigInt::multiply takes arguments x (a BigInt) and y (a
BigInt) and returns a BigInt. It performs the following steps when called:
Return x × y.
Note
Even if the result has a much larger bit width than the input, the exact
mathematical answer is given.
6.1.6.2.5 BigInt::divide ( x, y )
The abstract operation BigInt::divide takes arguments x (a BigInt) and y (a
BigInt) and returns either a normal completion
containing a BigInt or a throw completion. It
performs the following steps when called:
The abstract operation BigInt::remainder takes arguments n (a BigInt) and d (a
BigInt) and returns either a normal completion
containing a BigInt or a throw completion. It
performs the following steps when called:
The sign of the result is the sign of the dividend.
6.1.6.2.7 BigInt::add ( x, y )
The abstract operation BigInt::add takes arguments x (a BigInt) and y (a BigInt)
and returns a BigInt. It performs the following steps when called:
Return x + y.
6.1.6.2.8 BigInt::subtract ( x, y )
The abstract operation BigInt::subtract takes arguments x (a BigInt) and y (a
BigInt) and returns a BigInt. It performs the following steps when called:
Return x - y.
6.1.6.2.9 BigInt::leftShift ( x, y )
The abstract operation BigInt::leftShift takes arguments x (a BigInt) and y (a
BigInt) and returns a BigInt. It performs the following steps when called:
Semantics here should be equivalent to a bitwise shift, treating the BigInt
as an infinite length string of binary two's complement digits.
6.1.6.2.10 BigInt::signedRightShift ( x, y )
The abstract operation BigInt::signedRightShift takes arguments x (a BigInt) and
y (a BigInt) and returns a BigInt. It performs the following steps when called:
The abstract operation BigInt::unsignedRightShift takes arguments x (a BigInt) and
y (a BigInt) and returns a throw completion. It
performs the following steps when called:
Throw a TypeError exception.
6.1.6.2.12 BigInt::lessThan ( x, y )
The abstract operation BigInt::lessThan takes arguments x (a BigInt) and y (a
BigInt) and returns a Boolean. It performs the following steps when called:
If ℝ(x) < ℝ(y), return
true; otherwise return false.
6.1.6.2.13 BigInt::equal ( x, y )
The abstract operation BigInt::equal takes arguments x (a BigInt) and y (a
BigInt) and returns a Boolean. It performs the following steps when called:
If ℝ(x) = ℝ(y), return
true; otherwise return false.
6.1.6.2.14 BinaryAnd ( x, y )
The abstract operation BinaryAnd takes arguments x (0 or 1) and y (0 or 1) and
returns 0 or 1. It performs the following steps when called:
If x = 1 and y = 1, return 1.
Else, return 0.
6.1.6.2.15 BinaryOr ( x, y )
The abstract operation BinaryOr takes arguments x (0 or 1) and y (0 or 1) and
returns 0 or 1. It performs the following steps when called:
If x = 1 or y = 1, return 1.
Else, return 0.
6.1.6.2.16 BinaryXor ( x, y )
The abstract operation BinaryXor takes arguments x (0 or 1) and y (0 or 1) and
returns 0 or 1. It performs the following steps when called:
If x = 1 and y = 0, return 1.
Else if x = 0 and y = 1, return 1.
Else, return 0.
6.1.6.2.17 BigIntBitwiseOp ( op, x, y )
The abstract operation BigIntBitwiseOp takes arguments op (&,
^, or |), x (a BigInt), and y (a BigInt) and returns a
BigInt. It performs the following steps when called:
The abstract operation BigInt::bitwiseAND takes arguments x (a BigInt) and y (a
BigInt) and returns a BigInt. It performs the following steps when called:
The abstract operation BigInt::bitwiseXOR takes arguments x (a BigInt) and y (a
BigInt) and returns a BigInt. It performs the following steps when called:
The abstract operation BigInt::bitwiseOR takes arguments x (a BigInt) and y (a
BigInt) and returns a BigInt. It performs the following steps when called:
The abstract operation BigInt::toString takes arguments x (a BigInt) and radix
(an integer in the inclusive
interval from 2 to 36) and returns a String. It represents x as a String
using a positional numeral system with radix radix. The digits used in the representation of
a BigInt using radix r are taken from the first r code units of
"0123456789abcdefghijklmnopqrstuvwxyz" in order. The representation of BigInts other
than 0ℤ never includes leading zeroes. It performs the following steps
when called:
Return the String value consisting of the representation of x using radix
radix.
6.1.7 The Object Type
Each instance of the Object type, also
referred to simply as “an Object”, represents a collection of properties. Each property is either a data
property, or an accessor property:
A data property associates a key value with an
ECMAScript language value and a set of Boolean
attributes.
An accessor property associates a key value with
one or two accessor functions, and a set of Boolean attributes. The accessor functions are used to store
or retrieve an ECMAScript language value that is associated with
the property.
The properties of an object are uniquely identified using property keys. A property key is either
a String or a Symbol. All Strings and Symbols, including the empty String, are valid as property keys. A property name is a property key that is a String.
Property keys are used
to access properties and their values. There are two kinds of access for properties: get and
set, corresponding to value retrieval and assignment, respectively. The properties accessible via
get and set access includes both own properties that are a direct part of an object and
inherited properties which are provided by another associated object via a property inheritance
relationship. Inherited properties may be either own or inherited properties of the associated object. Each
own property of an object must each have a key value that is distinct from the key values of the other own
properties of that object.
All objects are logically collections of properties, but there are multiple forms of objects that differ in
their semantics for accessing and manipulating their properties. Please see 6.1.7.2 for definitions of the
multiple forms of objects.
In addition, some objects are callable; these are referred to as functions or function objects and are
described further below. All functions in ECMAScript are members of the Object type.
6.1.7.1 Property Attributes
Attributes are used in this specification to define and explain the state of Object properties as
described in Table 3. Unless specified explicitly, the
initial value of each attribute is its Default Value.
If the value is an
Object it must be a function object. The function's [[Call]] internal method (Table
5) is called with an empty arguments list to retrieve the property value each
time a get access of the property is performed.
If the value is an
Object it must be a function object. The function's [[Call]] internal method (Table
5) is called with an arguments list containing the assigned value as its sole
argument each time a set access of the property is performed. The effect of a property's [[Set]] internal method may, but is not required to, have an effect on the
value returned by subsequent calls to the property's [[Get]] internal
method.
If false, attempts to delete the property, change it from a data property
to an accessor
property or from an accessor property to a data
property, or make any changes to its attributes (other than replacing an
existing [[Value]] or setting [[Writable]] to
false) will fail.
6.1.7.2 Object Internal Methods and Internal Slots
The actual semantics of objects, in ECMAScript, are specified via algorithms called internal
methods. Each object in an ECMAScript engine is associated with a set of internal methods that
defines its runtime behaviour. These internal methods are not part of the ECMAScript language. They are
defined by this specification purely for expository purposes. However, each object within an
implementation of ECMAScript must behave as specified by the internal methods associated with it. The
exact manner in which this is accomplished is determined by the implementation.
Internal method names are polymorphic. This means that different object values may perform different
algorithms when a common internal method name is invoked upon them. That actual object upon which an
internal method is invoked is the “target” of the invocation. If, at runtime, the implementation of an
algorithm attempts to use an internal method of an object that the object does not support, a
TypeError exception is thrown.
Internal slots correspond to internal state that is associated with objects, Symbols, or Private Names and
used by various ECMAScript specification algorithms. Internal slots are not object properties and they are
not inherited. Depending upon the specific internal slot specification, such state may consist of values
of any ECMAScript language type or of specific
ECMAScript specification type values. Unless explicitly specified otherwise, internal slots are allocated
as part of the process of creating an object, Symbol, or Private Name and may not be dynamically
added. Unless specified otherwise, the initial value of an internal slot is the value
undefined. Various algorithms within this specification create values that have
internal slots. However, the ECMAScript language provides no direct way to manipulate internal slots.
All objects have an internal slot named [[PrivateElements]], which is a
List of PrivateElements. This List represents the values of the
private fields, methods, and accessors for the object. Initially, it is an empty List.
Internal methods and internal slots are identified within this specification using names enclosed in
double square brackets [[ ]].
Table 4 summarizes the essential internal
methods used by this specification that are applicable to all objects created or manipulated by
ECMAScript code. Every object must have algorithms for all of the essential internal methods. However, all
objects do not necessarily use the same algorithms for those methods.
An ordinary object is an object
that satisfies all of the following criteria:
For the internal methods listed in Table 4, the object uses those defined in
10.1.
If the object has a [[Call]] internal method, it uses either the one defined in
10.2.1 or the
one defined in 10.3.1.
If the object has a [[Construct]] internal method, it uses either the one
defined in 10.2.2 or
the one defined in 10.3.2.
An exotic object is an object that
is not an ordinary
object.
This specification recognizes different kinds of exotic objects by those objects' internal methods. An object that
is behaviourally equivalent to a particular kind of exotic object (such as an Array exotic object or a bound
function exotic object), but does not have the same collection of internal methods
specified for that kind, is not recognized as that kind of exotic object.
The “Signature” column of Table 4 and other similar tables describes the
invocation pattern for each internal method. The invocation pattern always includes a parenthesized list
of descriptive parameter names. If a parameter name is the same as an ECMAScript type name then the name
describes the required type of the parameter value. If an internal method explicitly returns a value, its
parameter list is followed by the symbol “→” and the type name of the returned value. The type names used
in signatures refer to the types defined in clause 6 augmented by the
following additional names. “any” means the value may be any ECMAScript
language type.
In addition to its parameters, an internal method always has access to the object that is the target of
the method invocation.
Determine the object that provides inherited properties for this object. A
null value indicates that there are no inherited properties.
[[SetPrototypeOf]]
(Object | Null) → Boolean
Associate this object with another object that provides inherited properties. Passing
null indicates that there are no inherited properties. Returns
true indicating that the operation was completed successfully or
false indicating that the operation was not successful.
[[IsExtensible]]
( ) → Boolean
Determine whether it is permitted to add additional properties to this object.
[[PreventExtensions]]
( ) → Boolean
Control whether new properties may be added to this object. Returns true if
the operation was successful or false if the operation was unsuccessful.
Return a Property Descriptor for
the own property of this object whose key is propertyKey, or
undefined if no such property exists.
[[DefineOwnProperty]]
(propertyKey, PropertyDescriptor) → Boolean
Create or alter the own property, whose key is propertyKey, to have the state
described by PropertyDescriptor. Return true if that property was
successfully created/updated or false if the property could not be created or
updated.
[[HasProperty]]
(propertyKey) → Boolean
Return a Boolean value indicating whether this object already has either an own or inherited
property whose key is propertyKey.
[[Get]]
(propertyKey, Receiver) →any
Return the value of the property whose key is propertyKey from this object. If any
ECMAScript code must be executed to retrieve the property value, Receiver is used as
the this value when evaluating the code.
[[Set]]
(propertyKey, value, Receiver) → Boolean
Set the value of the property whose key is propertyKey to value. If any
ECMAScript code must be executed to set the property value, Receiver is used as the
this value when evaluating the code. Returns true if the
property value was set or false if it could not be set.
[[Delete]]
(propertyKey) → Boolean
Remove the own property whose key is propertyKey from this object. Return
false if the property was not deleted and is still present. Return
true if the property was deleted or is not present.
Return a List whose elements are all of
the own property
keys for the object.
Table 5
summarizes additional essential internal methods that are supported by objects that may be called as
functions. A function object is
an object that supports the [[Call]] internal method. A constructor is an object that supports the [[Construct]] internal method. Every object that supports [[Construct]] must support [[Call]]; that is, every
constructor must be a
function
object. Therefore, a constructor may also be referred to as a constructor function or
constructorfunction
object.
Table 5: Additional Essential Internal Methods of Function Objects
Executes code associated with this object. Invoked via a function call expression. The arguments
to the internal method are a this value and a List whose elements are the
arguments passed to the function by a call expression. Objects that implement this internal
method are callable.
Creates an object. Invoked via the new operator or a super call. The
first argument to the internal method is a List whose elements are the
arguments of the constructor invocation or the super call.
The second argument is the object to which the new operator was initially applied.
Objects that implement this internal method are called constructors. A function
object is not necessarily a constructor and such non-constructorfunction
objects do not have a [[Construct]] internal method.
The semantics of the essential internal methods for ordinary objects and standard exotic objects are specified in clause 10. If any specified use of an
internal method of an exotic
object is not supported by an implementation, that usage must throw a
TypeError exception when attempted.
6.1.7.3 Invariants of the Essential Internal Methods
The Internal Methods of Objects of an ECMAScript engine must conform to the list of invariants specified
below. Ordinary ECMAScript Objects as well as all standard exotic objects in this specification maintain
these invariants. ECMAScript Proxy objects maintain these invariants by means of runtime checks on the
result of traps invoked on the [[ProxyHandler]] object.
Any implementation provided exotic objects must also maintain these invariants for those
objects. Violation of these invariants may cause ECMAScript code to have unpredictable behaviour and
create security issues. However, violation of these invariants must never compromise the memory safety of
an implementation.
An implementation must not allow these invariants to be circumvented in any manner such as by providing
alternative interfaces that implement the functionality of the essential internal methods without
enforcing their invariants.
Definitions:
The target of an internal method is the object upon which the internal method is called.
A target is non-extensible if it has been observed to return false from its
[[IsExtensible]] internal method, or true from its [[PreventExtensions]] internal method.
A non-existent property is a property that does not exist as an own property on a
non-extensible target.
All references to SameValue are according to the definition of the
SameValue algorithm.
Return value:
The value returned by any internal method must be a Completion Record with either:
[[Type]] = normal, [[Target]]
= empty, and [[Value]] = a value of the "normal return
type" shown below for that internal method, or
If target is non-extensible, and [[GetPrototypeOf]] returns a value
V, then any future calls to [[GetPrototypeOf]] should return the
SameValue as
V.
Note 2
An object's prototype chain should have finite length (that is, starting from any object, recursively
applying the [[GetPrototypeOf]] internal method to its result should
eventually lead to the value null). However, this requirement is not enforceable as
an object level invariant if the prototype chain includes any exotic objects that do not use the ordinary object
definition of [[GetPrototypeOf]]. Such a circular prototype chain may result
in infinite loops when accessing object properties.
[[SetPrototypeOf]] ( V )
The normal return type is Boolean.
If target is non-extensible, [[SetPrototypeOf]] must return
false, unless V is the SameValue as the target's observed [[GetPrototypeOf]] value.
[[IsExtensible]] ( )
The normal return type is Boolean.
If [[IsExtensible]] returns false, all future calls to [[IsExtensible]] on the target must return false.
[[PreventExtensions]] ( )
The normal return type is Boolean.
If [[PreventExtensions]] returns true, all future calls to
[[IsExtensible]] on the target must return false and the
target is now considered non-extensible.
If P is described as a non-configurable, non-writable own data property, all future calls to [[GetOwnProperty]] ( P ) must return Property Descriptor whose [[Value]] is SameValue as P's [[Value]]
attribute.
If P's attributes other than [[Writable]] and [[Value]] may change over time, or if the property might be deleted, then
P's [[Configurable]] attribute must be true.
If the [[Writable]] attribute may change from false to
true, then the [[Configurable]] attribute must be
true.
If the target is non-extensible and P is non-existent, then all future calls to [[GetOwnProperty]] (P) on the target must describe P as
non-existent (i.e. [[GetOwnProperty]] (P) must return
undefined).
Note 3
As a consequence of the third invariant, if a property is described as a data property and
it may return different values over time, then either or both of the [[Writable]] and [[Configurable]] attributes must be
true even if no mechanism to change the value is exposed via the other essential
internal methods.
[[DefineOwnProperty]] ( P, Desc )
The normal return type is Boolean.
[[DefineOwnProperty]] must return false if P has
previously been observed as a non-configurable own property of the target, unless either:
All attributes of Desc are the SameValue as P's attributes.
[[DefineOwnProperty]] (P, Desc) must return
false if target is non-extensible and P is a non-existent own property.
That is, a non-extensible target object cannot be extended with new properties.
[[HasProperty]] ( P )
The normal return type is Boolean.
If P was previously observed as a non-configurable own data or accessor property of
the target, [[HasProperty]] must return true.
If P was previously observed as a non-configurable, non-writable own data property of the
target with value V, then [[Get]] must return the SameValue as V.
If P was previously observed as a non-configurable own accessor property of the target whose [[Get]] attribute is undefined, the [[Get]] operation must return undefined.
[[Set]] ( P, V, Receiver )
The normal return type is Boolean.
If P was previously observed as a non-configurable, non-writable own data property of the
target, then [[Set]] must return false unless V
is the SameValue as
P's [[Value]] attribute.
If P was previously observed as a non-configurable own accessor property of the target whose [[Set]] attribute is undefined, the [[Set]] operation must return false.
[[Delete]] ( P )
The normal return type is Boolean.
If P was previously observed as a non-configurable own data or accessor property of
the target, [[Delete]] must return false.
The returned List must contain at least the keys of
all non-configurable own properties that have previously been observed.
If the target is non-extensible, the returned List must contain only
the keys of all own properties of the target that are observable using [[GetOwnProperty]].
The target must also have a [[Call]] internal method.
6.1.7.4 Well-Known Intrinsic Objects
Well-known intrinsics are built-in objects that are explicitly referenced by the algorithms of this
specification and which usually have realm-specific identities. Unless otherwise specified each intrinsic
object actually corresponds to a set of similar objects, one per realm.
Within this specification a reference such as %name% means the intrinsic object, associated with the
current realm, corresponding to the
name. A reference such as %name.a.b% means, as if the "b" property of the value of the
"a" property of the intrinsic object %name% was accessed prior to any ECMAScript code
being evaluated. Determination of the current realm and its intrinsics is described in 9.4. The
well-known intrinsics are listed in Table 6.
A specification type corresponds to meta-values that are used within algorithms to describe the semantics of
ECMAScript language constructs and ECMAScript language types. The specification types
include Reference Record, List, Completion Record, Property Descriptor, Environment
Record, Abstract Closure, and Data Block. Specification type values are
specification artefacts that do not necessarily correspond to any specific entity within an ECMAScript
implementation. Specification type values may be used to describe intermediate results of ECMAScript
expression evaluation but such values cannot be stored as properties of objects or values of ECMAScript
language variables.
6.2.1 The Enum Specification Type
Enums are values which are internal to the specification and
not directly observable from ECMAScript code. Enums are denoted using a sans-serif
typeface. For instance, a Completion Record's [[Type]] field takes on values like normal,
return, or throw. Enums have no characteristics other than
their name. The name of an enum serves no purpose other than to distinguish it from other enums, and implies
nothing about its usage or meaning in context.
6.2.2 The List and Record Specification Types
The List type is used to explain the evaluation of argument lists
(see 13.3.8)
in new expressions, in function calls, and in other algorithms where a simple ordered list of
values is needed. Values of the List type are simply ordered sequences of list elements containing the
individual values. These sequences may be of any length. The elements of a list may be randomly accessed
using 0-origin indices. For notational convenience an array-like syntax can be used to access List elements.
For example, arguments[2] is shorthand for saying the 3rd element of the List
arguments.
When an algorithm iterates over the elements of a List without specifying an order, the order used is the
order of the elements in the List.
For notational convenience within this specification, a literal syntax can be used to express a new List
value. For example, « 1, 2 » defines a List value that has two elements each of which is initialized to a
specific value. A new empty List can be expressed as « ».
In this specification, the phrase "the list-concatenation
of A, B, ..." (where each argument is a possibly empty List) denotes a new List value
whose elements are the concatenation of the elements (in order) of each of the arguments (in order).
As applied to a List of Strings, the phrase "sorted according to lexicographic code unit order" means sorting by the numeric value of each code unit up
to the length of the shorter string, and sorting the shorter string before the longer string if all are
equal, as described in the abstract operation IsLessThan.
The Record type is used to describe data aggregations within
the algorithms of this specification. A Record type value consists of one or more named fields. The value of
each field is an ECMAScript language value or specification value.
Field names are always enclosed in double brackets, for example [[Value]].
For notational convenience within this specification, an object literal-like syntax can be used to express
a Record value. For example, { [[Field1]]: 42, [[Field2]]:
false, [[Field3]]: empty } defines a
Record value that has three fields, each of which is initialized to a specific value. Field name order is
not significant. Any fields that are not explicitly listed are considered to be absent.
In specification text and algorithms, dot notation may be used to refer to a specific field of a Record
value. For example, if R is the record shown in the previous paragraph then R.[[Field2]] is shorthand for “the field of R named [[Field2]]”.
Schema for commonly used Record field combinations may be named, and that name may be used as a prefix to a
literal Record value to identify the specific kind of aggregations that is being described. For example:
PropertyDescriptor { [[Value]]: 42, [[Writable]]:
false, [[Configurable]]: true }.
6.2.3 The Set and Relation Specification Types
The Set type is used to explain a collection of unordered elements for use in the memory model. It is
distinct from the ECMAScript collection type of the same name. To disambiguate, instances of the ECMAScript
collection are consistently referred to as "Set objects" within this specification. Values of the Set type
are simple collections of elements, where no element appears more than once. Elements may be added to and
removed from Sets. Sets may be unioned, intersected, or subtracted from each other.
The Relation type is used to explain constraints on Sets.
Values of the Relation type are Sets of ordered pairs of values from its value domain. For example, a
Relation on events is a set of ordered pairs of events. For a Relation R and two values
a and b in the value domain of R, aRb is
shorthand for saying the ordered pair (a, b) is a member of R. A Relation
is the least Relation with respect to some conditions when it
is the smallest Relation that satisfies those conditions.
A strict partial order is a Relation value
R that satisfies the following.
For all a, b, and c in R's domain:
It is not the case that aRa, and
If aRb and bRc, then
aRc.
Note 1
The two properties above are called irreflexivity and transitivity, respectively.
A strict total order is a Relation value
R that satisfies the following.
For all a, b, and c in R's domain:
a is b or aRb or bRa, and
It is not the case that aRa, and
If aRb and bRc, then
aRc.
Note 2
The three properties above are called totality, irreflexivity, and transitivity, respectively.
6.2.4 The Completion Record Specification Type
The Completion Record specification type is used to
explain the runtime propagation of values and control flow such as the behaviour of statements
(break, continue, return and throw) that perform
nonlocal transfers of control.
Completion Records have the fields defined in Table 7.
The following shorthand terms are sometimes used to refer to Completion Records.
normal completion refers to any Completion Record
with a [[Type]] value of normal.
break completion refers to any Completion Record
with a [[Type]] value of break.
continue completion refers to any Completion
Record with a [[Type]] value of continue.
return completion refers to any Completion Record
with a [[Type]] value of return.
throw completion refers to any Completion Record
with a [[Type]] value of throw.
abrupt completion refers to any Completion Record
with a [[Type]] value other than normal.
a normal completion containing some
type of value refers to a normal completion that has a value of that type in its [[Value]] field.
Callable objects that are defined in this specification only return a normal completion or a throw
completion. Returning any other kind of Completion Record is considered an editorial error.
Implementation-defined callable objects must return either
a normal completion or a throw completion.
6.2.4.1 NormalCompletion ( value )
The abstract operation NormalCompletion takes argument value (any value except a Completion Record) and returns a
normal completion. It performs the
following steps when called:
The abstract operation ThrowCompletion takes argument value (an ECMAScript
language value) and returns a throw completion. It performs the
following steps when called:
The abstract operation ReturnCompletion takes argument value (an ECMAScript
language value) and returns a return completion. It performs the
following steps when called:
The abstract operation UpdateEmpty takes arguments completionRecord (a Completion Record) and
value (any value except a Completion Record) and
returns a Completion Record. It performs the
following steps when called:
The Reference Record type is used to explain the
behaviour of such operators as delete, typeof, the assignment operators, the
superkeyword and other language features. For example,
the left-hand operand of an assignment is expected to produce a Reference Record.
A Reference Record is a resolved name or (possibly not-yet-resolved) property binding; its fields are
defined by Table 8.
If not empty, the Reference Record represents a
property binding that was expressed using the superkeyword; it is called a Super Reference Record and its [[Base]] value will never be an Environment Record. In that
case, the [[ThisValue]] field holds the this value at
the time the Reference Record was created.
The following abstract operations are used in
this specification to operate upon Reference Records:
6.2.5.1 IsPropertyReference ( V )
The abstract operation IsPropertyReference takes argument V (a Reference Record) and returns a
Boolean. It performs the following steps when called:
If V.[[Base]] is unresolvable, return
false.
If V.[[Base]] is an Environment Record, return
false; otherwise return true.
6.2.5.2 IsUnresolvableReference ( V )
The abstract operation IsUnresolvableReference takes argument V (a Reference Record) and returns a
Boolean. It performs the following steps when called:
If V.[[Base]] is unresolvable, return
true; otherwise return false.
6.2.5.3 IsSuperReference ( V )
The abstract operation IsSuperReference takes argument V (a Reference Record) and returns a
Boolean. It performs the following steps when called:
If V.[[ThisValue]] is not empty, return
true; otherwise return false.
6.2.5.4 IsPrivateReference ( V )
The abstract operation IsPrivateReference takes argument V (a Reference Record) and returns a
Boolean. It performs the following steps when called:
If V.[[ReferencedName]] is a Private Name, return
true; otherwise return false.
Return ? base.GetBindingValue(V.[[ReferencedName]], V.[[Strict]]) (see
9.1).
Note
The object that may be created in step 3.a is not accessible outside of the above abstract
operation and the ordinary
object[[Get]] internal method. An implementation might
choose to avoid the actual creation of the object.
Return ? base.SetMutableBinding(V.[[ReferencedName]], W, V.[[Strict]]) (see 9.1).
Note
The object that may be created in step 3.a is not accessible outside of the above abstract
operation and the ordinary
object[[Set]] internal method. An implementation might
choose to avoid the actual creation of that object.
The abstract operation MakePrivateReference takes arguments baseValue (an ECMAScript
language value) and privateIdentifier (a String) and returns a Reference Record. It performs the
following steps when called:
Return the Reference Record { [[Base]]: baseValue, [[ReferencedName]]:
privateName, [[Strict]]: true, [[ThisValue]]: empty }.
6.2.6 The Property Descriptor Specification Type
The Property Descriptor type is used to explain
the manipulation and reification of Object property attributes. A Property Descriptor is a Record with zero or more fields, where
each field's name is an attribute name and its value is a corresponding attribute value as specified in
6.1.7.1. The schema name used within this specification
to tag literal descriptions of Property Descriptor records is “PropertyDescriptor”.
Property Descriptor values may be further classified as data Property Descriptors and accessor Property
Descriptors based upon the existence or use of certain fields. A data Property Descriptor is one that
includes any fields named either [[Value]] or [[Writable]]. An accessor Property Descriptor is one that includes any fields named
either [[Get]] or [[Set]]. Any Property Descriptor may
have fields named [[Enumerable]] and [[Configurable]]. A
Property Descriptor value may not be both a data Property Descriptor and an accessor Property Descriptor;
however, it may be neither (in which case it is a generic Property Descriptor). A fully
populated Property Descriptor is one that is either an accessor Property Descriptor or a data
Property Descriptor and that has all of the corresponding fields defined in Table
3.
The following abstract operations are used in
this specification to operate upon Property Descriptor values:
6.2.6.1 IsAccessorDescriptor ( Desc )
The abstract operation IsAccessorDescriptor takes argument Desc (a Property Descriptor) and returns a
Boolean. It performs the following steps when called:
If Desc has a [[Get]] field, return true.
If Desc has a [[Set]] field, return true.
Return false.
6.2.6.2 IsDataDescriptor ( Desc )
The abstract operation IsDataDescriptor takes argument Desc (a Property Descriptor) and returns a
Boolean. It performs the following steps when called:
If Desc has a [[Value]] field, return true.
If Desc has a [[Writable]] field, return
true.
Return false.
6.2.6.3 IsGenericDescriptor ( Desc )
The abstract operation IsGenericDescriptor takes argument Desc (a Property Descriptor) and returns a
Boolean. It performs the following steps when called:
The abstract operation FromPropertyDescriptor takes argument Desc (a Property Descriptor or
undefined) and returns an Object or undefined. It performs the
following steps when called:
If IsCallable(setter) is
false and setter is not undefined, throw a
TypeError exception.
Set desc.[[Set]] to setter.
If desc has a [[Get]] field or desc has a [[Set]] field, then
If desc has a [[Value]] field or desc has a [[Writable]] field, throw a TypeError exception.
Return desc.
6.2.6.6 CompletePropertyDescriptor ( Desc )
The abstract operation CompletePropertyDescriptor takes argument Desc (a Property Descriptor) and returns
unused. It performs the following steps when called:
Let like be the Record { [[Value]]: undefined, [[Writable]]:
false, [[Get]]: undefined, [[Set]]: undefined, [[Enumerable]]:
false, [[Configurable]]: false }.
If Desc does not have a [[Value]] field, set
Desc.[[Value]] to like.[[Value]].
If Desc does not have a [[Writable]] field, set
Desc.[[Writable]] to like.[[Writable]].
Else,
If Desc does not have a [[Get]] field, set
Desc.[[Get]] to like.[[Get]].
If Desc does not have a [[Set]] field, set
Desc.[[Set]] to like.[[Set]].
If Desc does not have an [[Enumerable]] field, set
Desc.[[Enumerable]] to like.[[Enumerable]].
If Desc does not have a [[Configurable]] field, set
Desc.[[Configurable]] to like.[[Configurable]].
Return unused.
6.2.7 The Environment Record Specification Type
The Environment
Record type is used to explain the behaviour of name resolution in nested functions and
blocks. This type and the operations upon it are defined in 9.1.
6.2.8 The Abstract Closure Specification Type
The Abstract Closure specification type is used to
refer to algorithm steps together with a collection of values. Abstract Closures are meta-values and are
invoked using function application style such as closure(arg1, arg2). Like
abstract operations, invocations
perform the algorithm steps described by the Abstract Closure.
In algorithm steps that create an Abstract Closure, values are captured with the verb "capture" followed by
a list of aliases. When an Abstract Closure is created, it captures the value that is associated with each
alias at that time. In steps that specify the algorithm to be performed when an Abstract Closure is called,
each captured value is referred to by the alias that was used to capture the value.
The Data Block specification type is used to describe a
distinct and mutable sequence of byte-sized (8 bit) numeric values. A byte value is an integer in the inclusive interval from 0 to 255. A Data Block value is
created with a fixed number of bytes that each have the initial value 0.
For notational convenience within this specification, an array-like syntax can be used to access the
individual bytes of a Data Block value. This notation presents a Data Block value as a 0-based
integer-indexed sequence of bytes. For example, if db is a 5 byte Data
Block value then db[2] can be used to access its 3rd byte.
A data block that resides in memory that can be referenced from multiple agents concurrently is designated a Shared Data Block. A Shared Data Block has an identity
(for the purposes of equality testing Shared Data Block values) that is address-free: it is tied
not to the virtual addresses the block is mapped to in any process, but to the set of locations in memory
that the block represents. Two data blocks are equal only if the sets of the locations they contain are
equal; otherwise, they are not equal and the intersection of the sets of locations they contain is empty.
Finally, Shared Data Blocks can be distinguished from Data Blocks.
Let db be a new Shared Data Block value consisting of size
bytes. If it is impossible to create such a Shared Data Block, throw a RangeError
exception.
The abstract operation CopyDataBlockBytes takes arguments toBlock (a Data Block or a
Shared Data
Block), toIndex (a non-negative integer), fromBlock (a Data Block or a Shared Data Block),
fromIndex (a non-negative integer), and count (a non-negative integer) and returns
unused. It performs the following steps when called:
Assert:
fromBlock and toBlock are distinct values.
Let bytes be a List whose
sole element is a nondeterministically chosen byte value.
NOTE: In implementations, bytes is the result of a non-atomic read instruction on
the underlying hardware. The nondeterminism is a semantic prescription of the memory
model to describe observable behaviour of hardware with weak consistency.
Let readEvent be ReadSharedMemory {
[[Order]]: unordered, [[NoTear]]: true, [[Block]]:
fromBlock, [[ByteIndex]]: fromIndex, [[ElementSize]]: 1 }.
Append readEvent to eventsRecord.[[EventList]].
Append Chosen Value Record { [[Event]]: readEvent, [[ChosenValue]]: bytes } to execution.[[ChosenValues]].
The PrivateElement type is a Record used in the specification of
private class fields, methods, and accessors. Although Property Descriptors are not used for
private elements, private fields behave similarly to non-configurable, non-enumerable, writable data properties, private
methods behave similarly to non-configurable, non-enumerable, non-writable data properties, and private accessors behave
similarly to non-configurable, non-enumerable accessor properties.
Values of the PrivateElement type are Record values whose fields
are defined by Table 9. Such values are referred to as PrivateElements.
6.2.11 The ClassFieldDefinition Record Specification Type
The ClassFieldDefinition type is a Record used in the
specification of class fields.
Values of the ClassFieldDefinition type are Record values whose fields
are defined by Table 10. Such values are referred to as ClassFieldDefinition Records.
The Private Name specification type is used to describe a
globally unique value (one which differs from any other Private Name, even if they are otherwise
indistinguishable) which represents the key of a private class element (field, method, or accessor). Each
Private Name has an immutable [[Description]] internal slot which is a String. A Private Name may be
installed on any ECMAScript object with PrivateFieldAdd or PrivateMethodOrAccessorAdd, and then read or
written using PrivateGet
and PrivateSet.
6.2.13 The ClassStaticBlockDefinition Record Specification Type
A ClassStaticBlockDefinition Record
is a Record value used to encapsulate the
executable code for a class static initialization block.
ClassStaticBlockDefinition Records have the fields listed in Table 11.
The function
object to be called during static initialization of a class.
7 Abstract Operations
These operations are not a part of the ECMAScript language; they are defined here solely to aid the
specification of the semantics of the ECMAScript language. Other, more specialized abstract operations are defined
throughout this specification.
7.1 Type Conversion
The ECMAScript language implicitly performs automatic type conversion as needed. To clarify the semantics of
certain constructs it is useful to define a set of conversion abstract operations. The conversion
abstract operations are polymorphic;
they can accept a value of any ECMAScript language type. But no other specification
types are used with these operations.
The BigInt type has no implicit conversions
in the ECMAScript language; programmers must call BigInt explicitly to convert values from other types.
7.1.1 ToPrimitive ( input [ , preferredType ] )
The abstract operation ToPrimitive takes argument input (an ECMAScript
language value) and optional argument preferredType
(string or number) and returns either a normal completion containing an
ECMAScript language value or a throw completion. It converts its
input argument to a non-Object type. If an object is capable of converting to more than
one primitive type, it may use the optional hint preferredType to favour that type. It performs
the following steps when called:
When ToPrimitive is called without a hint, then it generally behaves as if the hint were
number. However, objects may over-ride this behaviour by defining a %Symbol.toPrimitive% method. Of the objects defined in
this specification only Dates (see 21.4.4.45) and Symbol
objects (see 20.4.3.5) over-ride the default
ToPrimitive behaviour. Dates treat the absence of a hint as if the hint were
string.
The abstract operation ToBoolean takes argument argument (an ECMAScript
language value) and returns a Boolean. It converts argument to a value of type
Boolean. It performs the following steps when called:
The abstract operation ToNumeric takes argument value (an ECMAScript
language value) and returns either a normal completion containing either a
Number or a BigInt, or a throw completion. It returns
value converted to a Number or a BigInt. It performs the following steps when called:
The abstract operation RoundMVResult takes argument n (a mathematical
value) and returns a Number. It converts n to a Number in an implementation-defined manner. For the purposes of
this abstract operation, a digit is significant if it is not zero or there is a non-zero digit to its
left and there is a non-zero digit to its right. For the purposes of this abstract operation, "the
mathematical
value denoted by" a representation of a mathematical value is the inverse of "the
decimal representation of" a mathematical value. It performs the following steps when
called:
If the decimal representation of n has 20 or fewer significant digits, return 𝔽(n).
Let option1 be the mathematical value denoted by the result of replacing
each significant digit in the decimal representation of n after the 20th with a 0 digit.
Let option2 be the mathematical value denoted by the result of replacing
each significant digit in the decimal representation of n after the 20th with a 0 digit
and then incrementing it at the 20th position (with carrying as necessary).
The abstract operation ToIntegerOrInfinity takes argument argument (an ECMAScript
language value) and returns either a normal completion containing either an
integer, +∞, or -∞, or a
throw completion. It converts
argument to an integer
representing its Number value with fractional part truncated, or to +∞ or -∞ when that Number value is
infinite. It performs the following steps when called:
𝔽(ToIntegerOrInfinity(x))
never returns -0𝔽 for any value of x. The truncation of the
fractional part is performed after converting x to a mathematical value.
Step 5 is the only difference between ToUint32 and
ToInt32.
The ToUint32 abstract operation is idempotent: if applied to a result that it produced, the second
application leaves that value unchanged.
ToUint32(ToInt32(x)) is the same value as
ToUint32(x) for all values of x. (It is to preserve this latter property that
+∞𝔽 and -∞𝔽 are mapped to
+0𝔽.)
If f is even, return 𝔽(f); otherwise return 𝔽(f + 1).
Note
Unlike most other ECMAScript integer conversion operations, ToUint8Clamp rounds rather than
truncates non-integral values. It also uses “round half to even” tie-breaking, which differs from the
“round half up” tie-breaking of Math.round.
7.1.13 ToBigInt ( argument )
The abstract operation ToBigInt takes argument argument (an ECMAScript
language value) and returns either a normal completion containing a BigInt or
a throw completion. It converts
argument to a BigInt value, or throws if an implicit conversion from Number would be required. It
performs the following steps when called:
If argument is either undefined or null, throw a
TypeError exception.
If argumentis a Boolean, return a new Boolean
object whose [[BooleanData]] internal slot is set to argument. See
20.3
for a description of Boolean objects.
If argumentis a Number, return a new Number
object whose [[NumberData]] internal slot is set to argument. See
21.1 for
a description of Number objects.
If argumentis a String, return a new String
object whose [[StringData]] internal slot is set to argument. See
22.1 for
a description of String objects.
If argumentis a Symbol, return a new Symbol
object whose [[SymbolData]] internal slot is set to argument. See
20.4 for
a description of Symbol objects.
If argumentis a BigInt, return a new BigInt
object whose [[BigIntData]] internal slot is set to argument. See
21.2 for
a description of BigInt objects.
The abstract operation CanonicalNumericIndexString takes argument argument (a String) and
returns a Number or undefined. If argument is either "-0"
or exactly matches ToString(n) for some Number value n, it
returns the respective Number value. Otherwise, it returns undefined. It performs the
following steps when called:
The abstract operation IsCallable takes argument argument (an ECMAScript
language value) and returns a Boolean. It determines if argument is a callable
function with a [[Call]] internal method. It performs the following steps when
called:
If argument has a [[Call]] internal method, return
true.
Return false.
7.2.4 IsConstructor ( argument )
The abstract operation IsConstructor takes argument argument (an ECMAScript
language value) and returns a Boolean. It determines if argument is a
function object
with a [[Construct]] internal method. It performs the following steps when called:
If argument has a [[Construct]] internal method, return
true.
Return false.
7.2.5 IsExtensible ( O )
The abstract operation IsExtensible takes argument O (an Object) and returns either a normal completion containing a Boolean
or a throw completion. It is used to
determine whether additional properties can be added to O. It performs the following steps when
called:
The abstract operation IsStringWellFormedUnicode takes argument string (a String) and returns a
Boolean. It interprets string as a sequence of UTF-16 encoded code points, as described in
6.1.4, and determines whether it is a
well formed UTF-16 sequence.
It performs the following steps when called:
If cp.[[IsUnpairedSurrogate]] is true,
return false.
Set k to k + cp.[[CodeUnitCount]].
Return true.
7.2.8 SameType ( x, y )
The abstract operation SameType takes arguments x (an ECMAScript
language value) and y (an ECMAScript language value) and
returns a Boolean. It determines whether or not the two arguments are the same type. It performs the
following steps when called:
If x is undefined and y is undefined,
return true.
The abstract operation SameValue takes arguments x (an ECMAScript
language value) and y (an ECMAScript language value) and
returns a Boolean. It determines whether or not the two arguments are the same value. It performs the
following steps when called:
This algorithm differs from the IsStrictlyEqual Algorithm by treating all
NaN values as equivalent and by differentiating +0𝔽
from -0𝔽.
7.2.10 SameValueZero ( x, y )
The abstract operation SameValueZero takes arguments x (an ECMAScript
language value) and y (an ECMAScript language value) and
returns a Boolean. It determines whether or not the two arguments are the same value (ignoring the
difference between +0𝔽 and -0𝔽). It performs
the following steps when called:
SameValueZero differs from SameValue only in that it treats
+0𝔽 and -0𝔽 as equivalent.
7.2.11 SameValueNonNumber ( x, y )
The abstract operation SameValueNonNumber takes arguments x (an ECMAScript
language value, but not a Number) and y (an ECMAScript
language value, but not a Number) and returns a Boolean. It performs the following steps
when called:
For expository purposes, some cases are handled separately within this algorithm even if it is unnecessary
to do so.
Note 2
The specifics of what "x is y" means are detailed in 5.2.7.
7.2.12 IsLessThan ( x, y, LeftFirst )
The abstract operation IsLessThan takes arguments x (an ECMAScript
language value), y (an ECMAScript language value), and
LeftFirst (a Boolean) and returns either a normal completion containing either a
Boolean or undefined, or a throw completion. It
provides the semantics for the comparison x < y, returning true,
false, or undefined (which indicates that at least one operand is
NaN). The LeftFirst flag is used to control the order in which operations with
potentially visible side-effects are performed upon x and y. It is necessary because
ECMAScript specifies left to right evaluation of expressions. If LeftFirst is
true, the x parameter corresponds to an expression that occurs to the left of
the y parameter's corresponding expression. If LeftFirst is false,
the reverse is the case and operations must be performed upon y before x. It performs
the following steps when called:
If ℝ(nx) < ℝ(ny), return
true; otherwise return false.
Note 1
Step 3 differs from step 1.c in the algorithm that handles the addition
operator + (13.15.3) by using the logical-and
operation instead of the logical-or operation.
Note 2
The comparison of Strings uses a simple lexicographic ordering on sequences of UTF-16 code unit values.
There is no attempt to use the more complex, semantically oriented definitions of character or string
equality and collating order defined in the Unicode specification. Therefore String values that are
canonically equal according to the Unicode Standard but not in the same normalization form could test as
unequal. Also note that lexicographic ordering by code unit differs from ordering by code
point for Strings containing surrogate pairs.
If x is not finite or y is not finite, return false.
If ℝ(x) = ℝ(y), return
true; otherwise return false.
Return false.
7.2.14 IsStrictlyEqual ( x, y )
The abstract operation IsStrictlyEqual takes arguments x (an ECMAScript
language value) and y (an ECMAScript language value) and
returns a Boolean. It provides the semantics for the === operator. It performs the following
steps when called:
This algorithm differs from the SameValue Algorithm in its treatment of signed zeroes and NaNs.
7.3 Operations on Objects
7.3.1 MakeBasicObject ( internalSlotsList )
The abstract operation MakeBasicObject takes argument internalSlotsList (a List of internal slot names) and returns
an Object. It is the source of all ECMAScript objects that are created algorithmically, including both
ordinary objects
and exotic objects.
It factors out common steps used in creating all objects, and centralizes object creation. It performs the
following steps when called:
Set internalSlotsList to the list-concatenation of internalSlotsList and «
[[PrivateElements]] ».
Let obj be a newly created object with an internal slot for each name in
internalSlotsList.
Set obj's essential internal methods to the default ordinary object definitions specified in
10.1.
Assert: If the caller will
not be overriding both obj's [[GetPrototypeOf]] and [[SetPrototypeOf]] essential internal methods, then internalSlotsList
contains [[Prototype]].
Assert: If the caller will
not be overriding all of obj's [[SetPrototypeOf]], [[IsExtensible]], and [[PreventExtensions]] essential
internal methods, then internalSlotsList contains [[Extensible]].
If internalSlotsList contains [[Extensible]], set
obj.[[Extensible]] to true.
Return obj.
Note
Within this specification, exotic objects are created in abstract operations such as
ArrayCreate and
BoundFunctionCreate by first calling MakeBasicObject
to obtain a basic, foundational object, and then overriding some or all of that object's internal
methods. In order to encapsulate exotic object creation, the object's essential internal methods
are never modified outside those operations.
The abstract operation Set takes arguments O (an Object), P (a property key), V (an
ECMAScript language value), and Throw (a
Boolean) and returns either a normal completion containingunused or a throw completion. It is
used to set the value of a specific property of an object. V is the new value for the property.
It performs the following steps when called:
Let success be ? O.[[Set]](P, V, O).
If success is false and Throw is true,
throw a TypeError exception.
Let newDesc be the PropertyDescriptor { [[Value]]: V,
[[Writable]]: true, [[Enumerable]]:
true, [[Configurable]]: true }.
Return ? O.[[DefineOwnProperty]](P, newDesc).
Note
This abstract operation creates a property whose attributes are set to the same defaults used for
properties created by the ECMAScript language assignment operator. Normally, the property will not
already exist. If it does exist and is not configurable or if O is not extensible, [[DefineOwnProperty]] will return false.
7.3.6 CreateDataPropertyOrThrow ( O, P, V )
The abstract operation CreateDataPropertyOrThrow takes arguments O (an Object), P (a
property key), and
V (an ECMAScript language value) and returns either a
normal completion containingunused or a throw completion. It is
used to create a new own property of an object. It throws a TypeError exception if the
requested property update cannot be performed. It performs the following steps when called:
This abstract operation creates a property whose attributes are set to the same defaults used for
properties created by the ECMAScript language assignment operator. Normally, the property will not
already exist. If it does exist and is not configurable or if O is not extensible, [[DefineOwnProperty]] will return false causing this operation
to throw a TypeError exception.
7.3.7 CreateNonEnumerableDataPropertyOrThrow ( O, P,
V )
The abstract operation CreateNonEnumerableDataPropertyOrThrow takes arguments O (an Object),
P (a property
key), and V (an ECMAScript language value) and
returns unused. It is used to create a new non-enumerable own property of an
ordinary object.
It performs the following steps when called:
Assert: O is an
ordinary, extensible object with no non-configurable properties.
Let newDesc be the PropertyDescriptor { [[Value]]: V,
[[Writable]]: true, [[Enumerable]]:
false, [[Configurable]]: true }.
This abstract operation creates a property whose attributes are set to the same defaults used for
properties created by the ECMAScript language assignment operator except it is not enumerable. Normally,
the property will not already exist. If it does exist, DefinePropertyOrThrow is guaranteed
to complete normally.
7.3.8 DefinePropertyOrThrow ( O, P, desc )
The abstract operation DefinePropertyOrThrow takes arguments O (an Object), P (a
property key), and
desc (a Property Descriptor) and returns
either a normal completion containingunused or a throw completion. It is
used to call the [[DefineOwnProperty]] internal method of an object in a manner
that will throw a TypeError exception if the requested property update cannot be
performed. It performs the following steps when called:
Let success be ? O.[[DefineOwnProperty]](P, desc).
If success is false, throw a TypeError exception.
Return unused.
7.3.9 DeletePropertyOrThrow ( O, P )
The abstract operation DeletePropertyOrThrow takes arguments O (an Object) and P (a
property key) and
returns either a normal completion containingunused or a throw completion. It is
used to remove a specific own property of an object. It throws an exception if the property is not
configurable. It performs the following steps when called:
If func is either undefined or null, return
undefined.
If IsCallable(func) is false, throw
a TypeError exception.
Return func.
7.3.11 HasProperty ( O, P )
The abstract operation HasProperty takes arguments O (an Object) and P (a property key) and returns
either a normal completion containing a Boolean
or a throw completion. It is used to
determine whether an object has a property with the specified property key. The property may be either own or
inherited. It performs the following steps when called:
Return ? O.[[HasProperty]](P).
7.3.12 HasOwnProperty ( O, P )
The abstract operation HasOwnProperty takes arguments O (an Object) and P (a
property key) and
returns either a normal completion containing a Boolean
or a throw completion. It is used to
determine whether an object has an own property with the specified property key. It performs the following steps when
called:
The abstract operation Construct takes argument F (a constructor) and optional arguments
argumentsList (a List of ECMAScript
language values) and newTarget (a constructor) and returns either a normal completion containing an Object
or a throw completion. It is used to call the
[[Construct]] internal method of a function object. argumentsList and
newTarget are the values to be passed as the corresponding arguments of the internal method. If
argumentsList is not present, a new empty List is used as its value. If
newTarget is not present, F is used as its value. It performs the following steps when
called:
If newTarget is not present, set newTarget to F.
If argumentsList is not present, set argumentsList to a new empty List.
If newTarget is not present, this operation is equivalent to:
new F(...argumentsList)
7.3.15 SetIntegrityLevel ( O, level )
The abstract operation SetIntegrityLevel takes arguments O (an Object) and level
(sealed or frozen) and returns either a normal completion containing a Boolean
or a throw completion. It is used to fix the
set of own properties of an object. It performs the following steps when called:
The abstract operation TestIntegrityLevel takes arguments O (an Object) and level
(sealed or frozen) and returns either a normal completion containing a Boolean
or a throw completion. It is used to
determine if the set of own properties of an object are fixed. It performs the following steps when called:
NOTE: If the object is extensible, none of its properties are examined.
Let keys be ? O.[[OwnPropertyKeys]]().
For each element k of keys, do
Let currentDesc be ? O.[[GetOwnProperty]](k).
If currentDesc is not undefined, then
If currentDesc.[[Configurable]] is
true, return false.
If level is frozen and IsDataDescriptor(currentDesc) is
true, then
If currentDesc.[[Writable]] is
true, return false.
Return true.
7.3.17 CreateArrayFromList ( elements )
The abstract operation CreateArrayFromList takes argument elements (a List of ECMAScript
language values) and returns an Array. It is used to create an Array whose elements are
provided by elements. It performs the following steps when called:
The abstract operation LengthOfArrayLike takes argument obj (an Object) and returns either a
normal completion containing a
non-negative integer or a
throw completion. It returns the value
of the "length" property of an array-like object. It performs the following steps when
called:
The abstract operation CreateListFromArrayLike takes argument obj (an ECMAScript
language value) and optional argument validElementTypes
(all or property-key) and returns either a normal completion containing a List of ECMAScript
language values or a throw completion. It is
used to create a List value whose elements are provided by
the indexed properties of obj. validElementTypes indicates the types of values that
are allowed as elements. It performs the following steps when called:
If validElementTypes is not present, set validElementTypes to
all.
The abstract operation OrdinaryHasInstance takes arguments C (an ECMAScript
language value) and O (an ECMAScript language value) and
returns either a normal completion containing a Boolean
or a throw completion. It implements the
default algorithm for determining if O inherits from the instance object inheritance path
provided by C. It performs the following steps when called:
7.3.22 SpeciesConstructor ( O, defaultConstructor )
The abstract operation SpeciesConstructor takes arguments O (an Object) and
defaultConstructor (a constructor) and returns either a normal completion containing a constructor or a throw completion. It is used to retrieve
the constructor that
should be used to create new objects that are derived from O. defaultConstructor is
the constructor to use if
a constructor%Symbol.species% property cannot be found starting from
O. It performs the following steps when called:
The target passed in here is always a newly created object which is not directly accessible in case of
an error being thrown.
7.3.26 PrivateElementFind ( O, P )
The abstract operation PrivateElementFind takes arguments O (an Object) and P (a
Private Name)
and returns a PrivateElement or
empty. It performs the following steps when called:
If O.[[PrivateElements]] contains a PrivateElementpe such that
pe.[[Key]] is P, then
If entry is not empty, throw a TypeError
exception.
Append PrivateElement { [[Key]]: P, [[Kind]]:
field, [[Value]]: value } to
O.[[PrivateElements]].
Return unused.
7.3.28 PrivateMethodOrAccessorAdd ( O, method )
The abstract operation PrivateMethodOrAccessorAdd takes arguments O (an Object) and
method (a PrivateElement) and returns either a
normal completion containingunused or a throw completion. It
performs the following steps when called:
Assert:
method.[[Kind]] is either method or
accessor.
7.3.33 InitializeInstanceElements ( O, constructor )
The abstract operation InitializeInstanceElements takes arguments O (an Object) and
constructor (an ECMAScript function object) and returns either a normal completion containingunused or a throw completion. It
performs the following steps when called:
The abstract operation IteratorComplete takes argument iteratorResult (an Object) and returns
either a normal completion containing a Boolean
or a throw completion. It performs the
following steps when called:
The abstract operation IteratorStep takes argument iteratorRecord (an Iterator
Record) and returns either a normal completion
containing either an Object or done, or a throw completion. It requests the next
value from iteratorRecord.[[Iterator]] by calling
iteratorRecord.[[NextMethod]] and returns either
done indicating that the iterator has reached its end or the IteratorResult
object if a next value is available. It performs the following steps when called:
The abstract operation IteratorStepValue takes argument iteratorRecord (an Iterator
Record) and returns either a normal completion
containing either an ECMAScript language value or
done, or a throw completion. It
requests the next value from iteratorRecord.[[Iterator]] by calling
iteratorRecord.[[NextMethod]] and returns either
done indicating that the iterator has reached its end or the value from the
IteratorResult object if a next value is available.
It performs the following steps when called:
The abstract operation IteratorClose takes arguments iteratorRecord (an Iterator
Record) and completion (a Completion Record) and returns a
Completion Record. It is used to notify
an iterator that it should perform any actions it would
normally perform when it has reached its completed state. It performs the following steps when called:
The abstract operation AsyncIteratorClose takes arguments iteratorRecord (an Iterator
Record) and completion (a Completion Record) and returns a
Completion Record. It is used to notify
an async
iterator that it should perform any actions it would normally perform when it has reached
its completed state. It performs the following steps when called:
If innerResult.[[Value]]is not an Object, throw a
TypeError exception.
Return ? completion.
7.4.14 CreateIteratorResultObject ( value, done )
The abstract operation CreateIteratorResultObject takes arguments value (an ECMAScript
language value) and done (a Boolean) and returns an Object that conforms to
the IteratorResult interface. It creates an object that
conforms to the IteratorResult interface. It performs the following
steps when called:
The abstract operation CreateListIteratorRecord takes argument list (a List of ECMAScript
language values) and returns an Iterator Record. It creates an Iterator Record
whose [[NextMethod]] returns the successive elements of list. It
performs the following steps when called:
Let closure be a new Abstract Closure with no parameters that captures
list and performs the following steps when called:
The definitions for this operation are distributed over the "ECMAScript Language" sections of this
specification. Each definition appears after the defining occurrence of the relevant productions.
"*default*" is used within this specification as a synthetic name for a module's
default export when it does not have another name. An entry in the module's [[Environment]] is created with that name and holds the corresponding value, and
resolving the export named "default" by calling ResolveExport ( exportName [ ,
resolveSet ] ) for the module will return a ResolvedBinding
Record whose [[BindingName]] is
"*default*", which will then resolve in the module's [[Environment]] to the above-mentioned value. This is done only for ease of
specification, so that anonymous default exports can be resolved like any other export. This
"*default*" string is never accessible to ECMAScript code or to the module linking
algorithm.
It is defined piecewise over the following productions:
It is not necessary to treat export defaultAssignmentExpression as a constant declaration
because there is no syntax that permits assignment to the internal bound name used to reference a
module's default object.
8.2.4 Static Semantics: LexicallyDeclaredNames
The syntax-directed operation
LexicallyDeclaredNames takes no arguments and returns a List of Strings. It is defined piecewise
over the following productions:
The syntax-directed operation
LexicallyScopedDeclarations takes no arguments and returns a List of Parse Nodes.
It is defined piecewise over the following productions:
The syntax-directed operation
VarDeclaredNames takes no arguments and returns a List of Strings. It is
defined piecewise over the following productions:
The syntax-directed operation
VarScopedDeclarations takes no arguments and returns a List of Parse Nodes.
It is defined piecewise over the following productions:
The syntax-directed operation
TopLevelLexicallyDeclaredNames takes no arguments and returns a List of Strings. It is defined piecewise
over the following productions:
The syntax-directed operation
TopLevelLexicallyScopedDeclarations takes no arguments and returns a List of Parse Nodes.
It is defined piecewise over the following productions:
The syntax-directed operation
TopLevelVarDeclaredNames takes no arguments and returns a List of Strings. It is defined piecewise
over the following productions:
The syntax-directed operation
TopLevelVarScopedDeclarations takes no arguments and returns a List of Parse Nodes.
It is defined piecewise over the following productions:
The syntax-directed operation
ContainsDuplicateLabels takes argument labelSet (a List of Strings) and returns a Boolean. It
is defined piecewise over the following productions:
The syntax-directed operation
ContainsUndefinedBreakTarget takes argument labelSet (a List of Strings) and returns a Boolean. It
is defined piecewise over the following productions:
The syntax-directed operation
ContainsUndefinedContinueTarget takes arguments iterationSet (a List of Strings) and labelSet
(a List of Strings) and returns a Boolean. It
is defined piecewise over the following productions:
The abstract operation IsAnonymousFunctionDefinition takes argument expr (an AssignmentExpressionParse Node,
an InitializerParse Node,
or an ExpressionParse Node)
and returns a Boolean. It determines if its argument is a function definition that does not bind a name. It
performs the following steps when called:
The syntax-directed operation
ComputedPropertyContains takes argument symbol (a grammar symbol) and returns a Boolean. It is
defined piecewise over the following productions:
undefined is passed for environment to indicate that a PutValue operation should be
used to assign the initialization value. This is the case for var statements and formal
parameter lists of some non-strict functions (See 10.2.11). In those cases a lexical
binding is hoisted and preinitialized prior to evaluation of its initializer.
It is defined piecewise over the following productions:
When undefined is passed for environment it indicates that a PutValue operation should be
used to assign the initialization value. This is the case for formal parameter lists of non-strict
functions. In that case the formal parameter bindings are preinitialized in order to
deal with the possibility of multiple parameters with the same name.
It is defined piecewise over the following productions:
The syntax-directed operation
AssignmentTargetType takes no arguments and returns simple,
web-compat, or invalid. It is defined piecewise over the
following productions:
Environment Record is a specification type used to
define the association of Identifiers to
specific variables and functions, based upon the lexical nesting structure of ECMAScript code. Usually an
Environment Record is associated with some specific syntactic structure of ECMAScript code such as a FunctionDeclaration, a BlockStatement, or a Catch clause of a TryStatement. Each time such code is evaluated, a new Environment
Record is created to record the identifier bindings that are created by that code.
Every Environment Record has an [[OuterEnv]] field, which is either
null or a reference to an outer Environment Record. This is used to model the logical
nesting of Environment Record values. The outer reference of an (inner) Environment Record is a reference to
the Environment Record that logically surrounds the inner Environment Record. An outer Environment Record may,
of course, have its own outer Environment Record. An Environment Record may serve as the outer environment for
multiple inner Environment Records. For example, if a FunctionDeclaration contains two nested FunctionDeclarations then the Environment
Records of each of the nested functions will have as their outer Environment Record the Environment Record of
the current evaluation of the surrounding function.
Environment Records are purely specification mechanisms and need not correspond to any specific artefact of
an ECMAScript implementation. It is impossible for an ECMAScript program to directly access or manipulate such
values.
A Function Environment Record
corresponds to the invocation of an ECMAScript function object, and contains
bindings for the top-level declarations within that function. It may establish a new
this binding. It also captures the state necessary to support super
method invocations.
An Object Environment Record is used
to define the effect of ECMAScript elements such as WithStatement that associate identifier bindings with
the properties of some object.
A Global Environment Record is used
for Script global declarations. It does
not have an outer environment; its [[OuterEnv]] is null.
It may be prepopulated with identifier bindings and it includes an associated global object
whose properties provide some of the global environment's identifier bindings. As ECMAScript code is
executed, additional properties may be added to the global object and the initial
properties may be modified.
The Environment
Record abstract class includes the abstract specification methods defined in Table 14. These abstract methods
have distinct concrete algorithms for each of the concrete subclasses.
Determine if an Environment Record has a binding for the String
value N. Return true if it does and false if it
does not.
CreateMutableBinding(N, D)
Create a new but uninitialized mutable binding in an Environment Record. The String
value N is the text of the bound name. If the Boolean argument D is
true the binding may be subsequently deleted.
CreateImmutableBinding(N, S)
Create a new but uninitialized immutable binding in an Environment Record. The String
value N is the text of the bound name. If S is true then
attempts to set it after it has been initialized will always throw an exception, regardless of the
strict mode setting of operations that reference that binding.
InitializeBinding(N, V)
Set the value of an already existing but uninitialized binding in an Environment
Record. The String value N is the text of the bound name.
V is the value for the binding and is a value of any ECMAScript language type.
SetMutableBinding(N, V, S)
Set the value of an already existing mutable binding in an Environment
Record. The String value N is the text of the bound name.
V is the value for the binding and may be a value of any ECMAScript language type. Sis a Boolean flag. If
S is true and the binding cannot be set throw a
TypeError exception.
GetBindingValue(N, S)
Returns the value of an already existing binding from an Environment Record. The String
value N is the text of the bound name. S is used to identify references
originating in strict mode code or that otherwise require strict
mode reference semantics. If S is true and the binding does not
exist throw a ReferenceError exception. If the binding exists but is
uninitialized a ReferenceError is thrown, regardless of the value of
S.
DeleteBinding(N)
Delete a binding from an Environment Record. The String value
N is the text of the bound name. If a binding for N exists, remove the
binding and return true. If the binding exists but cannot be removed return
false. If the binding does not exist return true.
HasThisBinding()
Determine if an Environment Record establishes a
this binding. Return true if it does and false
if it does not.
HasSuperBinding()
Determine if an Environment Record establishes a
super method binding. Return true if it does and
false if it does not. If it returns true it implies that the
Environment Record is a Function Environment Record, although
the reverse implication does not hold.
WithBaseObject()
If this Environment Record is associated with a
with statement, return the with object. Otherwise, return
undefined.
9.1.1.1 Declarative Environment Records
Each Declarative Environment Record
is associated with an ECMAScript program scope containing variable, constant, let, class, module, import,
and/or function declarations. A Declarative Environment Record binds the set of identifiers defined by the
declarations contained within its scope.
9.1.1.1.1 HasBinding ( N )
The HasBinding concrete method of a Declarative Environment
RecordenvRec takes argument N (a String) and returns a
normal completion containing a
Boolean. It determines if the argument identifier is one of the identifiers bound by the record. It
performs the following steps when called:
If envRec has a binding for N, return true.
Return false.
9.1.1.1.2 CreateMutableBinding ( N, D )
The CreateMutableBinding concrete method of a Declarative Environment
RecordenvRec takes arguments N (a String) and D (a
Boolean) and returns a normal completion containingunused. It creates a new mutable binding for the name N that is
uninitialized. A binding must not already exist in this Environment Record for N.
If D is true, the new binding is marked as being subject to deletion. It
performs the following steps when called:
Assert: envRec
does not already have a binding for N.
Create a mutable binding in envRec for N and record that it is
uninitialized. If D is true, record that the newly created binding may
be deleted by a subsequent DeleteBinding call.
Return unused.
9.1.1.1.3 CreateImmutableBinding ( N, S )
The CreateImmutableBinding concrete method of a Declarative Environment
RecordenvRec takes arguments N (a String) and S (a
Boolean) and returns a normal completion containingunused. It creates a new immutable binding for the name N that is
uninitialized. A binding must not already exist in this Environment Record for N.
If S is true, the new binding is marked as a strict binding. It performs
the following steps when called:
Assert: envRec
does not already have a binding for N.
Create an immutable binding in envRec for N and record that it is
uninitialized. If S is true, record that the newly created binding is
a strict binding.
Return unused.
9.1.1.1.4 InitializeBinding ( N, V )
The InitializeBinding concrete method of a Declarative Environment
RecordenvRec takes arguments N (a String) and V (an
ECMAScript language value) and returns a
normal completion containingunused. It is used to set the bound value of the current binding of the
identifier whose name is N to the value V. An uninitialized binding for
N must already exist. It performs the following steps when called:
Assert: envRec
must have an uninitialized binding for N.
Set the bound value for N in envRec to V.
Record that the binding for N in envRec has been
initialized.
Return unused.
9.1.1.1.5 SetMutableBinding ( N, V, S )
The SetMutableBinding concrete method of a Declarative Environment
RecordenvRec takes arguments N (a String), V (an
ECMAScript language value), and S (a
Boolean) and returns either a normal completion
containingunused or a throw completion. It attempts to
change the bound value of the current binding of the identifier whose name is N to the value
V. A binding for N normally already exists, but in rare cases it may not. If the
binding is an immutable binding, a TypeError is thrown if S is
true. It performs the following steps when called:
If envRec does not have a binding for
N, then
If S is true, throw a ReferenceError
exception.
Perform ! envRec.CreateMutableBinding(N, true).
Perform ! envRec.InitializeBinding(N, V).
Return unused.
If the binding for N in envRec is a strict binding, set S to
true.
If the binding for N in envRec has not yet been initialized, then
Throw a ReferenceError exception.
Else if the binding for N in envRec is a mutable binding, then
Change its bound value to V.
Else,
Assert: This is an
attempt to change the value of an immutable binding.
If S is true, throw a TypeError exception.
Return unused.
Note
An example of ECMAScript code that results in a missing binding at step 1 is:
functionf() { eval("var x; x = (delete x, 0);"); }
9.1.1.1.6 GetBindingValue ( N, S )
The GetBindingValue concrete method of a Declarative Environment
RecordenvRec takes arguments N (a String) and S (a
Boolean) and returns either a normal completion
containing an ECMAScript language value or a throw completion. It returns the
value of its bound identifier whose name is N. If the binding exists but is uninitialized a
ReferenceError is thrown, regardless of the value of S. It performs the
following steps when called:
If the binding for N in envRec is an uninitialized binding, throw a
ReferenceError exception.
Return the value currently bound to N in envRec.
9.1.1.1.7 DeleteBinding ( N )
The DeleteBinding concrete method of a Declarative Environment
RecordenvRec takes argument N (a String) and returns a
normal completion containing a
Boolean. It can only delete bindings that have been explicitly designated as being subject to deletion.
It performs the following steps when called:
If the binding for N in envRec cannot be deleted, return
false.
Remove the binding for N from envRec.
Return true.
9.1.1.1.8 HasThisBinding ( )
The HasThisBinding concrete method of a Declarative Environment
RecordenvRec takes no arguments and returns false. It
performs the following steps when called:
The HasSuperBinding concrete method of a Declarative Environment
RecordenvRec takes no arguments and returns false. It
performs the following steps when called:
The WithBaseObject concrete method of a Declarative Environment
RecordenvRec takes no arguments and returns undefined.
It performs the following steps when called:
Return undefined.
9.1.1.2 Object Environment Records
Each Object Environment Record is
associated with an object called its binding object. An Object Environment Record binds the set
of string identifier names that directly correspond to the property names of its binding object. Property keys that are not
strings in the form of an IdentifierName are not included in the set of bound
identifiers. Both own and inherited properties are included in the set regardless of the setting of their
[[Enumerable]] attribute. Because properties can be dynamically added and deleted
from objects, the set of identifiers bound by an Object Environment Record may potentially change as a
side-effect of any operation that adds or deletes properties. Any bindings that are created as a result of
such a side-effect are considered to be a mutable binding even if the Writable attribute of the
corresponding property is false. Immutable bindings do not exist for Object Environment
Records.
Object Environment Records created for with statements (14.11) can provide their binding object as an
implicit this value for use in function calls. The capability is controlled by a
Boolean [[IsWithEnvironment]] field.
Object Environment Records have the additional state fields listed in Table 15.
Indicates whether this Environment Record is created for a
with statement.
9.1.1.2.1 HasBinding ( N )
The HasBinding concrete method of an Object Environment RecordenvRec
takes argument N (a String) and returns either a normal completion containing a
Boolean or a throw completion. It determines if
its associated binding object has a property whose name is N. It performs the following steps
when called:
Let bindingObject be envRec.[[BindingObject]].
Let foundBinding be ? HasProperty(bindingObject,
N).
If foundBinding is false, return false.
If envRec.[[IsWithEnvironment]] is false,
return true.
The CreateMutableBinding concrete method of an Object Environment RecordenvRec takes arguments N (a String) and D (a Boolean) and returns
either a normal completion containingunused or a throw completion. It
creates in an Environment Record's associated binding object a
property whose name is N and initializes it to the value undefined. If
D is true, the new property's [[Configurable]]
attribute is set to true; otherwise it is set to false. It
performs the following steps when called:
Let bindingObject be envRec.[[BindingObject]].
Perform ? DefinePropertyOrThrow(bindingObject, N,
PropertyDescriptor { [[Value]]: undefined, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: D }).
Return unused.
Note
Normally envRec will not have a binding for N but if it does, the semantics
of DefinePropertyOrThrow may result in an existing
binding being replaced or shadowed or cause an abrupt completion to be
returned.
9.1.1.2.3 CreateImmutableBinding ( N, S )
The CreateImmutableBinding concrete method of an Object Environment Record is
never used within this specification.
In this specification, all uses of CreateMutableBinding for Object Environment Records are immediately
followed by a call to InitializeBinding for the same name. Hence, this specification does not
explicitly track the initialization state of bindings in Object Environment Records.
9.1.1.2.5 SetMutableBinding ( N, V, S )
The SetMutableBinding concrete method of an Object Environment RecordenvRec takes arguments N (a String), V (an ECMAScript language value), and S (a
Boolean) and returns either a normal completion
containingunused or a throw completion. It attempts to set
the value of the Environment Record's associated binding object's
property whose name is N to the value V. A property named N normally
already exists but if it does not or is not currently writable, error handling is determined by
S. It performs the following steps when called:
Let bindingObject be envRec.[[BindingObject]].
Let stillExists be ? HasProperty(bindingObject,
N).
If stillExists is false and S is true,
throw a ReferenceError exception.
The GetBindingValue concrete method of an Object Environment RecordenvRec takes arguments N (a String) and S (a Boolean) and returns
either a normal completion containing an
ECMAScript language value or a throw completion. It returns the
value of its associated binding object's property whose name is N. The property should
already exist but if it does not the result depends upon S. It performs the following steps
when called:
The DeleteBinding concrete method of an Object Environment RecordenvRec takes argument N (a String) and returns either a normal completion containing a
Boolean or a throw completion. It can only delete
bindings that correspond to properties of the environment object whose [[Configurable]] attribute have the value true. It performs the
following steps when called:
Let bindingObject be envRec.[[BindingObject]].
Return ? bindingObject.[[Delete]](N).
9.1.1.2.8 HasThisBinding ( )
The HasThisBinding concrete method of an Object Environment RecordenvRec takes no arguments and returns false. It performs the following
steps when called:
The HasSuperBinding concrete method of an Object Environment RecordenvRec takes no arguments and returns false. It performs the following
steps when called:
The WithBaseObject concrete method of an Object Environment RecordenvRec takes no arguments and returns an Object or undefined. It performs
the following steps when called:
If envRec.[[IsWithEnvironment]] is true,
return envRec.[[BindingObject]].
Otherwise, return undefined.
9.1.1.3 Function Environment Records
A Function Environment Record is a
Declarative Environment Record that is used
to represent the top-level scope of a function and, if the function is not an ArrowFunction, provides a this binding. If a
function is not an ArrowFunction
function and references super, its Function Environment Record also contains the state that
is used to perform super method invocations from within the function.
Function Environment Records have the additional state fields listed in Table 16.
If this Environment Record was created by the [[Construct]] internal method, [[NewTarget]] is
the value of the [[Construct]]newTarget parameter.
Otherwise, its value is undefined.
Function Environment Records support all of the Declarative Environment
Record methods listed in Table 14 and share the same
specifications for all of those methods except for HasThisBinding and HasSuperBinding. In addition,
Function Environment Records support the methods listed in Table 17:
Assert:
envRec.[[ThisBindingStatus]] is not
lexical.
If envRec.[[ThisBindingStatus]] is
initialized, throw a ReferenceError exception.
Set envRec.[[ThisValue]] to V.
Set envRec.[[ThisBindingStatus]] to
initialized.
Return unused.
9.1.1.3.2 HasThisBinding ( )
The HasThisBinding concrete method of a Function Environment RecordenvRec takes no arguments and returns a Boolean. It performs the following steps when called:
If envRec.[[ThisBindingStatus]] is
lexical, return false; otherwise return
true.
9.1.1.3.3 HasSuperBinding ( )
The HasSuperBinding concrete method of a Function Environment RecordenvRec takes no arguments and returns a Boolean. It performs the following steps when called:
If envRec.[[ThisBindingStatus]] is
lexical, return false.
If envRec.[[FunctionObject]].[[HomeObject]] is undefined, return
false; otherwise return true.
Assert:
envRec.[[ThisBindingStatus]] is not
lexical.
If envRec.[[ThisBindingStatus]] is
uninitialized, throw a ReferenceError exception.
Return envRec.[[ThisValue]].
9.1.1.3.5 GetSuperBase ( envRec )
The abstract operation GetSuperBase takes argument envRec (a Function Environment Record) and returns an
Object, null, or undefined. It returns the object that is the base
for super property accesses bound in envRec. The value
undefined indicates that such accesses will produce runtime errors. It performs the
following steps when called:
Let home be envRec.[[FunctionObject]].[[HomeObject]].
A Global Environment Record is used to
represent the outer most scope that is shared by all of the ECMAScript Script elements that are processed in a common realm. A Global Environment Record provides
the bindings for built-in globals (clause 19), properties of the global object, and for all top-level
declarations (8.2.9, 8.2.11) that occur within a
Script.
The HasBinding concrete method of a Global Environment RecordenvRec
takes argument N (a String) and returns either a normal completion containing a
Boolean or a throw completion. It determines if
the argument identifier is one of the identifiers bound by the record. It performs the following steps
when called:
Let DclRec be envRec.[[DeclarativeRecord]].
If ! DclRec.HasBinding(N) is true, return
true.
Let ObjRec be envRec.[[ObjectRecord]].
Return ? ObjRec.HasBinding(N).
9.1.1.4.2 CreateMutableBinding ( N, D )
The CreateMutableBinding concrete method of a Global Environment RecordenvRec takes arguments N (a String) and D (a Boolean) and returns
either a normal completion containingunused or a throw completion. It
creates a new mutable binding for the name N that is uninitialized. The binding is created in
the associated DeclarativeRecord. A binding for N must not already exist in the
DeclarativeRecord. If D is true, the new binding is marked as being
subject to deletion. It performs the following steps when called:
Let DclRec be envRec.[[DeclarativeRecord]].
If ! DclRec.HasBinding(N) is true, throw a
TypeError exception.
Return ! DclRec.CreateMutableBinding(N, D).
9.1.1.4.3 CreateImmutableBinding ( N, S )
The CreateImmutableBinding concrete method of a Global Environment RecordenvRec takes arguments N (a String) and S (a Boolean) and returns
either a normal completion containingunused or a throw completion. It
creates a new immutable binding for the name N that is uninitialized. A binding must not
already exist in this Environment Record for N. If S
is true, the new binding is marked as a strict binding. It performs the following
steps when called:
Let DclRec be envRec.[[DeclarativeRecord]].
If ! DclRec.HasBinding(N) is true, throw a
TypeError exception.
Return ! DclRec.CreateImmutableBinding(N, S).
9.1.1.4.4 InitializeBinding ( N, V )
The InitializeBinding concrete method of a Global Environment RecordenvRec takes arguments N (a String) and V (an ECMAScript language value) and returns either a
normal completion containingunused or a throw completion. It
is used to set the bound value of the current binding of the identifier whose name is N to
the value V. An uninitialized binding for N must already exist. It performs the
following steps when called:
The SetMutableBinding concrete method of a Global Environment RecordenvRec takes arguments N (a String), V (an ECMAScript language value), and S (a
Boolean) and returns either a normal completion
containingunused or a throw completion. It attempts to
change the bound value of the current binding of the identifier whose name is N to the value
V. If the binding is an immutable binding and S is true, a
TypeError is thrown. A property named N normally already exists but if it
does not or is not currently writable, error handling is determined by S. It performs the
following steps when called:
Let DclRec be envRec.[[DeclarativeRecord]].
If ! DclRec.HasBinding(N) is true, then
Return ? DclRec.SetMutableBinding(N, V, S).
Let ObjRec be envRec.[[ObjectRecord]].
Return ? ObjRec.SetMutableBinding(N,
V, S).
9.1.1.4.6 GetBindingValue ( N, S )
The GetBindingValue concrete method of a Global Environment RecordenvRec takes arguments N (a String) and S (a Boolean) and returns
either a normal completion containing an
ECMAScript language value or a throw completion. It returns the
value of its bound identifier whose name is N. If the binding is an uninitialized binding
throw a ReferenceError exception. A property named N normally already
exists but if it does not or is not currently writable, error handling is determined by S. It
performs the following steps when called:
Let DclRec be envRec.[[DeclarativeRecord]].
If ! DclRec.HasBinding(N) is true, then
Return ? DclRec.GetBindingValue(N, S).
Let ObjRec be envRec.[[ObjectRecord]].
Return ? ObjRec.GetBindingValue(N,
S).
9.1.1.4.7 DeleteBinding ( N )
The DeleteBinding concrete method of a Global Environment RecordenvRec takes argument N (a String) and returns either a normal completion containing a
Boolean or a throw completion. It can only delete
bindings that have been explicitly designated as being subject to deletion. It performs the following
steps when called:
The HasThisBinding concrete method of a Global Environment RecordenvRec takes no arguments and returns true. It performs the following
steps when called:
The HasSuperBinding concrete method of a Global Environment RecordenvRec takes no arguments and returns false. It performs the following
steps when called:
The WithBaseObject concrete method of a Global Environment RecordenvRec takes no arguments and returns undefined. It performs the following
steps when called:
The abstract operation HasLexicalDeclaration takes arguments envRec (a Global
Environment Record) and N (a String) and returns a Boolean. It determines
if the argument identifier has a binding in envRec that was created using a lexical
declaration such as a LexicalDeclaration or a ClassDeclaration. It performs the following steps when
called:
Let DclRec be envRec.[[DeclarativeRecord]].
Return ! DclRec.HasBinding(N).
9.1.1.4.13 HasRestrictedGlobalProperty ( envRec, N )
The abstract operation HasRestrictedGlobalProperty takes arguments envRec (a Global
Environment Record) and N (a String) and returns either a normal completion containing a
Boolean or a throw completion. It determines if
the argument identifier is the name of a property of the global object that must not be shadowed by
a global lexical binding. It performs the following steps when called:
Let ObjRec be envRec.[[ObjectRecord]].
Let globalObject be ObjRec.[[BindingObject]].
Let existingProp be ? globalObject.[[GetOwnProperty]](N).
If existingProp is undefined, return false.
If existingProp.[[Configurable]] is true,
return false.
Return true.
Note
Properties may exist upon a global object that were directly created rather than
being declared using a var or function declaration. A global lexical binding may not be created that
has the same name as a non-configurable property of the global object. The global property
"undefined" is an example of such a property.
9.1.1.4.14 CanDeclareGlobalVar ( envRec, N )
The abstract operation CanDeclareGlobalVar takes arguments envRec (a Global
Environment Record) and N (a String) and returns either a normal completion containing a
Boolean or a throw completion. It determines if a
corresponding CreateGlobalVarBinding call would succeed if
called for the same argument N. Redundant var declarations and var declarations for
pre-existing global
object properties are allowed. It performs the following steps when called:
Let existingProp be ? globalObject.[[GetOwnProperty]](N).
If existingProp is undefined, return ? IsExtensible(globalObject).
If existingProp.[[Configurable]] is true,
return true.
If IsDataDescriptor(existingProp) is
true and existingProp has attribute values { [[Writable]]: true, [[Enumerable]]: true }, return
true.
Return false.
9.1.1.4.16 CreateGlobalVarBinding ( envRec, N,
D )
The abstract operation CreateGlobalVarBinding takes arguments envRec (a Global
Environment Record), N (a String), and D (a Boolean) and
returns either a normal completion containingunused or a throw completion. It
creates and initializes a mutable binding in the associated Object
Environment Record. If a binding already exists, it is reused and assumed to be
initialized. It performs the following steps when called:
Global function declarations are always represented as own properties of the global
object. If possible, an existing own property is reconfigured to have a standard
set of attribute values. Step 7 is equivalent to
what calling the InitializeBinding concrete method would do and if globalObject is a
Proxy will produce the same sequence of Proxy trap calls.
9.1.1.5 Module Environment Records
A Module Environment Record is a Declarative Environment Record that is used
to represent the outer scope of an ECMAScript Module. In additional to normal mutable and immutable bindings,
Module Environment Records also provide immutable import bindings which are bindings that provide indirect
access to a target binding that exists in another Environment Record.
Module Environment Records support all of the Declarative Environment
Record methods listed in Table 14 and share the same
specifications for all of those methods except for GetBindingValue, DeleteBinding, HasThisBinding and
GetThisBinding. In addition, Module Environment Records support the methods listed in Table 20:
The GetBindingValue concrete method of a Module Environment RecordenvRec takes arguments N (a String) and S (a Boolean) and returns
either a normal completion containing an
ECMAScript language value or a throw completion. It returns the
value of its bound identifier whose name is N. However, if the binding is an indirect binding
the value of the target binding is returned. If the binding exists but is uninitialized a
ReferenceError is thrown. It performs the following steps when called:
The HasThisBinding concrete method of a Module Environment RecordenvRec takes no arguments and returns true. It performs the following
steps when called:
9.1.1.5.5 CreateImportBinding ( envRec, N,
M, N2 )
The abstract operation CreateImportBinding takes arguments envRec (a Module
Environment Record), N (a String), M (a Module
Record), and N2 (a String) and returns unused. It
creates a new initialized immutable indirect binding for the name N. A binding must not
already exist in envRec for N. N2 is the name of a binding that exists
in M's Module Environment Record. Accesses to the
value of the new binding will indirectly access the bound value of the target binding. It performs the
following steps when called:
Assert: envRec
does not already have a binding for N.
Assert: When
M.[[Environment]] is instantiated, it will have a direct binding
for N2.
Create an immutable indirect binding in envRec for N that references
M and N2 as its target binding and record that the binding is initialized.
The abstract operation NewObjectEnvironment takes arguments O (an Object), W (a
Boolean), and E (an Environment Record or null) and
returns an Object Environment Record. It performs the
following steps when called:
The abstract operation NewFunctionEnvironment takes arguments F (an ECMAScript function object) and
newTarget (an Object or undefined) and returns a Function Environment Record. It performs the
following steps when called:
If F.[[ThisMode]] is lexical, set
env.[[ThisBindingStatus]] to lexical.
Else, set env.[[ThisBindingStatus]] to
uninitialized.
Set env.[[NewTarget]] to newTarget.
Set env.[[OuterEnv]] to F.[[Environment]].
Return env.
9.1.2.5 NewGlobalEnvironment ( G, thisValue )
The abstract operation NewGlobalEnvironment takes arguments G (an Object) and
thisValue (an Object) and returns a Global Environment Record. It
performs the following steps when called:
The abstract operation ResolvePrivateIdentifier takes arguments privateEnv (a PrivateEnvironment
Record) and identifier (a String) and returns a Private Name. It
performs the following steps when called:
Before it is evaluated, all ECMAScript code must be associated with a realm. Conceptually, a realm consists of a set of intrinsic objects, an ECMAScript global
environment, all of the ECMAScript code that is loaded within the scope of that global environment, and other
associated state and resources.
A realm is represented in this
specification as a Realm Record with the
fields specified in Table 22:
Template objects are canonicalized separately for each realm using its Realm Record's [[TemplateMap]].
Each [[Site]] value is a Parse Node that is a TemplateLiteral. The associated
[[Array]] value is the corresponding template object that is passed to a
tag function.
Note 1
Once a Parse Node becomes unreachable, the
corresponding [[Array]] is also unreachable, and it would be
unobservable if an implementation removed the pair from the [[TemplateMap]] list.
A map from the specifier strings imported by this realm to the resolved Module
Record. The list does not contain two different Recordsr1 and
r2 such that ModuleRequestsEqual(r1,
r2) is true.
Field reserved for use by hosts
that need to associate additional information with a Realm Record.
9.3.1 InitializeHostDefinedRealm ( )
The abstract operation InitializeHostDefinedRealm takes no arguments and returns either a normal completion containingunused or a throw completion. It
performs the following steps when called:
Set fields of realmRec.[[Intrinsics]] with the values listed in Table 6. The field names are the names
listed in column one of the table. The value of each field is a new object value fully and recursively
populated with property values as defined by the specification of each object in clauses 19 through
28. All object
property values are newly created object values. All values that are built-in function objects are
created by performing CreateBuiltinFunction(steps,
length, name, slots, realmRec, prototype) where
steps is the definition of that function provided by this specification, name is
the initial value of the function's "name" property, length is the initial
value of the function's "length" property, slots is a list of the names,
if any, of the function's specified internal slots, and prototype is the specified value of
the function's [[Prototype]] internal slot. The creation of the intrinsics and
their properties must be ordered to avoid any dependencies upon objects that have not yet been created.
Let desc be the fully populated data Property Descriptor for the
property, containing the specified attributes for the property. For properties listed in 19.2, 19.3, or 19.4 the value of the [[Value]] attribute is the corresponding intrinsic object from
realmRec.
An execution context is a specification device that is
used to track the runtime evaluation of code by an ECMAScript implementation. At any point in time, there is
at most one execution context per agent
that is actually executing code. This is known as the agent's running execution context. All references to the
running
execution context in this specification denote the running execution
context of the surrounding agent.
The execution context
stack is used to track execution contexts. The running execution context is always the
top element of this stack. A new execution context is created whenever control is transferred from the
executable code associated with the currently running execution context to executable code that is not
associated with that execution context. The newly created execution context is pushed onto the stack and
becomes the running execution context.
An execution context contains whatever implementation specific state is necessary to track the execution
progress of its associated code. Each execution context has at least the state components listed in Table 23.
Table 23: State Components for All Execution Contexts
Component
Purpose
code evaluation state
Any state needed to perform, suspend, and resume evaluation of the code associated with this
execution
context.
Evaluation of code by the
running
execution context may be suspended at various points defined within this specification.
Once the running execution context has been suspended a different
execution context may become the running execution context and commence evaluating its
code. At some later time a suspended execution context may again become the running execution
context and continue evaluating its code at the point where it had previously been
suspended. Transition of the running execution context status among execution contexts
usually occurs in stack-like last-in/first-out manner. However, some ECMAScript features require non-LIFO
transitions of the running execution context.
In most situations only the running execution context (the top of the execution context
stack) is directly manipulated by algorithms within this specification. Hence when the
terms “LexicalEnvironment”, and “VariableEnvironment” are used without qualification they are in reference to
those components of the running execution context.
An execution context is purely a specification mechanism and need not correspond to any particular artefact
of an ECMAScript implementation. It is impossible for ECMAScript code to directly access or observe an
execution context.
9.4.1 GetActiveScriptOrModule ( )
The abstract operation GetActiveScriptOrModule takes no arguments and returns a Script Record, a Module
Record, or null. It is used to determine the running script or module,
based on the running execution context. It performs the following
steps when called:
If no such execution context exists, return
null; otherwise return ec's ScriptOrModule.
9.4.2 ResolveBinding ( name [ , env ] )
The abstract operation ResolveBinding takes argument name (a String) and optional argument
env (an Environment Record or undefined) and
returns either a normal completion containing a Reference Record or a throw completion. It is used to
determine the binding of name. env can be used to explicitly provide the Environment
Record that is to be searched for the binding. It performs the following steps when
called:
The result of ResolveBinding is always a Reference Record
whose [[ReferencedName]] field is name.
9.4.3 GetThisEnvironment ( )
The abstract operation GetThisEnvironment takes no arguments and returns an Environment
Record. It finds the Environment Record that currently supplies the binding of
the keywordthis. It performs the
following steps when called:
The abstract operation GetNewTarget takes no arguments and returns an Object or
undefined. It determines the NewTarget value using the LexicalEnvironment of the
running
execution context. It performs the following steps when called:
The abstract operation GetGlobalObject takes no arguments and returns an Object. It returns the global object used by
the currently running execution context. It performs the following
steps when called:
A Job is an Abstract Closure with no parameters that
initiates an ECMAScript computation when no other ECMAScript computation is currently in progress.
At some future point in time, when there is no running context in the agent for which the job is scheduled and that agent's execution context
stack is empty, the implementation must:
Host
environments are not required to treat Jobs uniformly with respect to scheduling. For example, web browsers and
Node.js treat Promise-handling Jobs as a
higher priority than other work; future features may add Jobs that are not treated at such a high priority.
At any particular time, scriptOrModule (a Script Record, a Module Record, or
null) is the active script or module
if all of the following conditions are true:
The specific choice of Realm is up
to the host
environment. This initial execution context and Realm is only in use before any callback function is
invoked. When a callback function related to a Job, like a Promise handler, is invoked, the invocation pushes its own
execution
context and Realm.
Particular kinds of Jobs have additional
conformance requirements.
The WHATWG HTML specification (https://html.spec.whatwg.org/), for example, uses the
host-defined value
to propagate the incumbent settings object for Promise callbacks.
JobCallback Records have the fields listed in Table 26.
An implementation of HostMakeJobCallback must conform to the following requirements:
It must return a JobCallback Record whose [[Callback]] field is callback.
The default implementation of HostMakeJobCallback performs the following steps when called:
Return the JobCallback Record { [[Callback]]: callback, [[HostDefined]]:
empty }.
ECMAScript hosts that are not web
browsers must use the default implementation of HostMakeJobCallback.
Note
This is called at the time that the callback is passed to the function that is responsible for its
being eventually scheduled and run. For example, promise.then(thenAction) calls
MakeJobCallback on thenAction at the time of invoking Promise.prototype.then,
not at the time of scheduling the reaction Job.
ECMAScript hosts that are not web
browsers must use the default implementation of HostCallJobCallback.
9.5.4 HostEnqueueGenericJob ( job, realm )
The host-defined
abstract operation HostEnqueueGenericJob takes arguments job (a JobAbstract Closure) and realm (a Realm Record) and returns
unused. It schedules job in the realmrealm in the agent signified by realm.[[AgentSignifier]] to be performed at some future time. The Abstract
Closures used with this algorithm are intended to be scheduled without additional
constraints, such as priority and ordering.
An implementation of HostEnqueueGenericJob must conform to the requirements in 9.5.
9.5.5 HostEnqueuePromiseJob ( job, realm )
The host-defined
abstract operation HostEnqueuePromiseJob takes arguments job (a JobAbstract Closure) and realm (a Realm Record or
null) and returns unused. It schedules job to be
performed at some future time. The Abstract Closures used with this algorithm are intended to
be related to the handling of Promises, or otherwise, to be scheduled with equal priority to Promise
handling operations.
An implementation of HostEnqueuePromiseJob must conform to the requirements in 9.5 as well as the following:
Let scriptOrModule be GetActiveScriptOrModule() at the time
HostEnqueuePromiseJob is invoked. If realm is not null, each time
job is invoked the implementation must perform implementation-defined steps such that
scriptOrModule is the active script or module at the time of
job's invocation.
Jobs must run in the same order as
the HostEnqueuePromiseJob invocations that scheduled them.
Note
The realm for Jobs
returned by NewPromiseResolveThenableJob is usually the
result of calling GetFunctionRealm on the thenfunction object. The
realm for Jobs returned by
NewPromiseReactionJob is usually the result of
calling GetFunctionRealm on the handler if the handler is not
undefined. If the handler is undefined, realm is
null. For both kinds of Jobs, when GetFunctionRealm completes abnormally (i.e. called on a
revoked Proxy), realm is the current Realm Record at the time of the GetFunctionRealm
call. When the realm is null, no user ECMAScript code will be evaluated
and no new ECMAScript objects (e.g. Error objects) will be created. The WHATWG HTML specification (https://html.spec.whatwg.org/), for example, uses
realm to check for the ability to run script and for the entry concept.
The host-defined
abstract operation HostEnqueueTimeoutJob takes arguments timeoutJob (a JobAbstract Closure), realm (a Realm Record), and
milliseconds (a non-negative finite Number) and returns unused. It schedules
timeoutJob in the realmrealm in the agent
signified by realm.[[AgentSignifier]] to be performed after at least
milliseconds milliseconds.
An implementation of HostEnqueueTimeoutJob must conform to the requirements in 9.5.
The default value computed for the isLittleEndian parameter when it is needed by the
algorithms GetValueFromBuffer and SetValueInBuffer. The choice is implementation-defined and should be the
alternative that is most efficient for the implementation. Once the value has been observed it
cannot change.
Initially 0, used to assign unique incrementing values to the [[AsyncEvaluationOrder]] field of modules that are asynchronous or have
asynchronous dependencies.
Once the values of [[Signifier]], [[IsLockFree1]], and [[IsLockFree2]] have been observed by any agent in the agent cluster they cannot change.
Note 2
The values of [[IsLockFree1]] and [[IsLockFree2]] are
not necessarily determined by the hardware, but may also reflect implementation choices that can vary over
time and between ECMAScript implementations.
There is no [[IsLockFree4]] field: 4-byte atomic operations are always
lock-free.
In practice, if an atomic operation is implemented with any type of lock the operation is not lock-free.
Lock-free does not imply wait-free: there is no upper bound on how many machine steps may be required to
complete a lock-free atomic operation.
That an atomic access of size n is lock-free does not imply anything about the (perceived)
atomicity of non-atomic accesses of size n, specifically, non-atomic accesses may still be
performed as a sequence of several separate memory accesses. See ReadSharedMemory and WriteSharedMemory for details.
Note 3
An agent is a specification
mechanism and need not correspond to any particular artefact of an ECMAScript implementation.
9.6.1 AgentSignifier ( )
The abstract operation AgentSignifier takes no arguments and returns an agent signifier. It performs the following steps when
called:
In some environments it may not be reasonable for a given agent to suspend. For example, in a web browser environment, it may be
reasonable to disallow suspending a document's main event handling thread, while still allowing workers'
event handling threads to suspend.
9.6.3 IncrementModuleAsyncEvaluationCount ( )
The abstract operation IncrementModuleAsyncEvaluationCount takes no arguments and returns an integer. It performs the following steps
when called:
Set AR.[[ModuleAsyncEvaluationCount]] to count + 1.
Return count.
Note
This value is only used to keep track of the relative evaluation order between pending modules. An
implementation may unobservably reset [[ModuleAsyncEvaluationCount]] to 0
whenever there are no pending modules.
9.7 Agent Clusters
An agent cluster is a maximal set of agents that can communicate by operating on
shared memory.
Note 1
Programs within different agents
may share memory by unspecified means. At a minimum, the backing memory for SharedArrayBuffers can be
shared among the agents in the
cluster.
There may be agents that can
communicate by message passing that cannot share memory; they are never in the same agent cluster.
The agents in a cluster need not
all be alive at some particular point in time. If agentA creates another agentB, after which A terminates and B creates
agentC, the three agents are in the same cluster if A
could share some memory with B and B could share some memory with C.
All agents within a cluster must have
the same value for the [[LittleEndian]] field in their respective Agent Records.
Note 3
If different agents within an
agent cluster have different values of [[LittleEndian]] it becomes hard to use
shared memory for multi-byte data.
All agents within a cluster must have
the same values for the [[IsLockFree1]] field in their respective Agent Records; similarly for the
[[IsLockFree2]] field.
All agents within a cluster must have
different values for the [[Signifier]] field in their respective Agent Records.
An embedding may deactivate (stop forward progress) or activate (resume forward progress) an agent without the agent's knowledge or cooperation. If the embedding does so,
it must not leave some agents in the
cluster active while other agents in
the cluster are deactivated indefinitely.
Note 4
The purpose of the preceding restriction is to avoid a situation where an agent deadlocks or starves because another agent has been deactivated. For example, if
an HTML shared worker that has a lifetime independent of documents in any windows were allowed to share
memory with the dedicated worker of such an independent document, and the document and its dedicated
worker were to be deactivated while the dedicated worker holds a lock (say, the document is pushed into
its window's history), and the shared worker then tries to acquire the lock, then the shared worker will
be blocked until the dedicated worker is activated again, if ever. Meanwhile other workers trying to
access the shared worker from other windows will starve.
The implication of the restriction is that it will not be possible to share memory between agents that don't belong to the same
suspend/wake collective within the embedding.
An embedding may terminate an agent
without any of the agent's cluster's
other agents' prior knowledge or
cooperation. If an agent is terminated
not by programmatic action of its own or of another agent in the cluster but by forces external to the cluster, then the
embedding must choose one of two strategies: Either terminate all the agents in the cluster, or provide reliable APIs that allow
the agents in the cluster to coordinate
so that at least one remaining member of the cluster will be able to detect the termination, with the
termination data containing enough information to identify the agent that was terminated.
Note 5
Examples of that type of termination are: operating systems or users terminating agents that are running in separate processes; the
embedding itself terminating an agent that is running in-process with the other agents when per-agent resource accounting indicates that the agent is runaway.
Each of the following specification values, and values transitively reachable from them, belong to exactly
one agent cluster.
An agent cluster is a specification mechanism and need not correspond to any particular artefact of an
ECMAScript implementation.
9.8 Forward Progress
For an agent to make forward
progress is for it to perform an evaluation step according to this specification.
An agent becomes blocked when
its running
execution context waits synchronously and indefinitely for an external event. Only
agents whose Agent Record's [[CanBlock]] field is true can become blocked in this sense. An
unblockedagent is one that is
not blocked.
Implementations must ensure that:
every unblocked agent with a
dedicated executing
thread eventually makes forward progress
an agent does not cause another
agent to become blocked except via
explicit APIs that provide blocking.
Note
This, along with the liveness guarantee in the memory model, ensures that all seq-cst
writes eventually become observable to all agents.
9.9 Processing Model of WeakRef and FinalizationRegistry Targets
9.9.1 Objectives
This specification does not make any guarantees that any object or symbol will be garbage collected.
Objects or symbols which are not live may be released after long periods of time, or never at all.
For this reason, this specification uses the term "may" when describing behaviour triggered by garbage
collection.
The semantics of WeakRefs and FinalizationRegistrys is based on two
operations which happen at particular points in time:
When WeakRef.prototype.deref is called, the referent (if undefined is not
returned) is kept alive so that subsequent, synchronous accesses also return the same value. This list is
reset when synchronous work is done using the ClearKeptObjects abstract operation.
Some ECMAScript implementations include garbage collector implementations which run in the background,
including when ECMAScript is idle. Letting the host environment schedule CleanupFinalizationRegistry allows it to resume
ECMAScript execution in order to run finalizer work, which may free up held values, reducing overall memory
usage.
9.9.2 Liveness
For some set of objects and/or symbols S a hypothetical
WeakRef-oblivious execution with respect to S is an execution whereby the abstract
operation WeakRefDeref of a WeakRef whose referent is an element of
S always returns undefined.
Note 1
WeakRef-obliviousness, together with liveness, capture
two notions. One, that a WeakRef itself does not keep its referent alive. Two,
that cycles in liveness does not imply that a value is live. To be concrete, if determining v's
liveness depends on determining the liveness of a WeakRef referent, r,
r's liveness cannot assume v's liveness, which would be circular reasoning.
Note 2
WeakRef-obliviousness is defined on sets of objects or
symbols instead of individual values to account for cycles. If it were defined on individual values, then
a WeakRef referent in a cycle will be considered live
even though its identity is only observed via other WeakRef referents in the cycle.
Note 3
Colloquially, we say that an individual object or symbol is live if every set containing it is live.
At any point during evaluation, a set of objects and/or symbols S is considered live if either of the following conditions is met:
Any element in S is included in any agent's [[KeptAlive]]List.
There exists a valid future hypothetical WeakRef-oblivious execution with respect to S that
observes the identity of any value in S.
Note 4
The second condition above intends to capture the intuition that a value is live if its identity is
observable via non-WeakRef means. A value's identity may be observed by
observing a strict equality comparison or observing the value being used as key in a Map.
Note 5
Presence of an object or a symbol in a field, an internal slot, or a property does not imply that the
value is live. For example if the value in question is never passed back to the program, then it cannot
be observed.
This is the case for keys in a WeakMap, members of a WeakSet, as well as the [[WeakRefTarget]] and [[UnregisterToken]] fields of a
FinalizationRegistry Cell record.
The above definition implies that, if a key in a WeakMap is not live, then its corresponding value is
not necessarily live either.
Note 6
Liveness is the lower bound for guaranteeing which WeakRefs engines must not empty.
Liveness as defined here is undecidable. In practice, engines use conservative approximations such as
reachability. There is expected to be significant implementation leeway.
9.9.3 Execution
At any time, if a set of objects and/or symbols S is not live, an ECMAScript implementation may perform the
following steps atomically:
For each element value of S, do
For each WeakRefref such that
ref.[[WeakRefTarget]] is value, do
Set ref.[[WeakRefTarget]] to empty.
For each FinalizationRegistryfg
such that fg.[[Cells]] contains a Recordcell such that
cell.[[WeakRefTarget]] is value, do
For each WeakMap map such that map.[[WeakMapData]]
contains a Recordr such that
r.[[Key]] is value, do
Set r.[[Key]] to empty.
Set r.[[Value]] to empty.
For each WeakSet set such that set.[[WeakSetData]]
contains value, do
Replace the element of set.[[WeakSetData]] whose value is
value with an element whose value is empty.
Note 1
Together with the definition of liveness, this clause prescribes optimizations that an implementation
may apply regarding WeakRefs.
It is possible to access an object without observing its identity. Optimizations such as dead variable
elimination and scalar replacement on properties of non-escaping objects whose identity is not observed
are allowed. These optimizations are thus allowed to observably empty WeakRefs that point to such objects.
On the other hand, if an object's identity is observable, and that object is in the [[WeakRefTarget]] internal slot of a WeakRef, optimizations such as
rematerialization that observably empty the WeakRef are prohibited.
Implementations are not obligated to empty WeakRefs for maximal sets of non-live objects or symbols.
If an implementation chooses a non-live set S in which to empty WeakRefs, this definition requires that it empties
WeakRefs for all values in S
simultaneously. In other words, it is not conformant for an implementation to empty a WeakRef pointing to a value v without
emptying out other WeakRefs that, if not emptied, could result in an
execution that observes the value of v.
The host-defined
abstract operation HostEnqueueFinalizationRegistryCleanupJob takes argument
finalizationRegistry (a FinalizationRegistry) and
returns unused.
Let cleanupJob be a new JobAbstract
Closure with no parameters that captures finalizationRegistry and performs
the following steps when called:
An implementation of HostEnqueueFinalizationRegistryCleanupJob schedules cleanupJob to be
performed at some future time, if possible. It must also conform to the requirements in 9.5.
9.10 ClearKeptObjects ( )
The abstract operation ClearKeptObjects takes no arguments and returns unused.
ECMAScript implementations are expected to call ClearKeptObjects when a synchronous sequence of ECMAScript
executions completes. It performs the following steps when called:
When the abstract operation AddToKeptObjects is called with a target object or symbol, it adds the target to
a list that will point strongly at the target until ClearKeptObjects is called.
Assert:
finalizationRegistry has [[Cells]] and [[CleanupCallback]] internal slots.
Let callback be finalizationRegistry.[[CleanupCallback]].
While finalizationRegistry.[[Cells]] contains a Recordcell such that
cell.[[WeakRefTarget]] is empty, an
implementation may perform the following steps:
The abstract operation CanBeHeldWeakly takes argument v (an ECMAScript
language value) and returns a Boolean. It returns true if and only if
v is suitable for use as a weak reference. Only values that are suitable for use as a weak
reference may be a key of a WeakMap, an element of a WeakSet, the target of a WeakRef,
or one of the targets of a FinalizationRegistry. It performs the
following steps when called:
A language value without language
identity can be manifested without prior reference and is unsuitable for use as a weak
reference. A Symbol value produced by Symbol.for, unlike other Symbol values, does not have language
identity and is unsuitable for use as a weak reference. Well-known symbols are likely to never be
collected, but are nonetheless treated as suitable for use as a weak reference because they are limited in
number and therefore manageable by a variety of implementation approaches. However, any value associated
to a well-known symbol in a live WeakMap is unlikely to be collected and could “leak” memory
resources in implementations.
10 Ordinary and Exotic Objects Behaviours
10.1 Ordinary Object Internal Methods and Internal Slots
All ordinary
objects have an internal slot called [[Prototype]]. The value of
this internal slot is either null or an object and is used for implementing inheritance.
Assume a property named P is missing from an ordinary objectO but exists on its [[Prototype]] object. If P refers to a data property on the [[Prototype]] object, O inherits it for get access, making it behave as if
P was a property of O. If P refers to a writable data property on the [[Prototype]] object, set access of P on O creates a new data property named
P on O. If P refers to a non-writable data property on the [[Prototype]] object, set access of P on O fails. If P
refers to an accessor
property on the [[Prototype]] object, the accessor is inherited by
O for both get access and set access.
Every ordinary
object has a Boolean-valued [[Extensible]] internal slot which is
used to fulfill the extensibility-related internal method invariants specified in 6.1.7.3. Namely, once the value of
an object's [[Extensible]] internal slot has been set to false, it
is no longer possible to add properties to the object, to modify the value of the object's [[Prototype]] internal slot, or to subsequently change the value of [[Extensible]] to true.
Each ordinary
object internal method delegates to a similarly-named abstract operation. If such an
abstract operation depends on another internal method, then the internal method is invoked on O
rather than calling the similarly-named abstract operation directly. These semantics ensure that exotic objects have their
overridden internal methods invoked when ordinary object internal methods are applied to them.
10.1.1[[GetPrototypeOf]] ( )
The [[GetPrototypeOf]] internal method of an ordinary objectO takes no arguments
and returns a normal completion containing either an
Object or null. It performs the following steps when called:
The abstract operation OrdinaryGetPrototypeOf takes argument O (an Object) and returns an
Object or null. It performs the following steps when called:
Return O.[[Prototype]].
10.1.2[[SetPrototypeOf]] ( V )
The [[SetPrototypeOf]] internal method of an ordinary objectO takes argument
V (an Object or null) and returns a normal completion containing a Boolean.
It performs the following steps when called:
The abstract operation OrdinarySetPrototypeOf takes arguments O (an Object) and V
(an Object or null) and returns a Boolean. It performs the following steps when called:
If p.[[GetPrototypeOf]] is not the ordinary
object internal method defined in 10.1.1,
set done to true.
Else, set p to p.[[Prototype]].
Set O.[[Prototype]] to V.
Return true.
Note
The loop in step 7 guarantees that there will be no cycles
in any prototype chain that only includes objects that use the ordinary object definitions for [[GetPrototypeOf]] and [[SetPrototypeOf]].
10.1.3[[IsExtensible]] ( )
The [[IsExtensible]] internal method of an ordinary objectO takes no arguments
and returns a normal completion containing a Boolean.
It performs the following steps when called:
The abstract operation OrdinaryIsExtensible takes argument O (an Object) and returns a
Boolean. It performs the following steps when called:
Return O.[[Extensible]].
10.1.4[[PreventExtensions]] ( )
The [[PreventExtensions]] internal method of an ordinary objectO takes no arguments
and returns a normal completion containingtrue. It performs the following steps when called:
The abstract operation OrdinaryGetOwnProperty takes arguments O (an Object) and P
(a property key) and
returns a Property Descriptor or
undefined. It performs the following steps when called:
If O does not have an own property with key P, return
undefined.
10.1.6.2 IsCompatiblePropertyDescriptor ( Extensible,
Desc, Current )
The abstract operation IsCompatiblePropertyDescriptor takes arguments Extensible (a Boolean),
Desc (a Property Descriptor), and
Current (a Property Descriptor or
undefined) and returns a Boolean. It performs the following steps when called:
10.1.6.3 ValidateAndApplyPropertyDescriptor ( O, P,
extensible, Desc, current )
The abstract operation ValidateAndApplyPropertyDescriptor takes arguments O (an Object or
undefined), P (a property key), extensible (a Boolean), Desc
(a Property Descriptor), and
current (a Property Descriptor or
undefined) and returns a Boolean. It returns true if and only if
Desc can be applied as the property of an object with specified extensibility and
current property current while upholding invariants. When such
application is possible and O is not undefined, it is performed for the
property named P (which is created if necessary). It performs the following steps when called:
Create an own accessor property named P of object
O whose [[Get]], [[Set]], [[Enumerable]], and [[Configurable]] attributes
are set to the value of the corresponding field in Desc if Desc has that
field, or to the attribute's default value
otherwise.
Else,
Create an own data property named P of object
O whose [[Value]], [[Writable]],
[[Enumerable]], and [[Configurable]]
attributes are set to the value of the corresponding field in Desc if
Desc has that field, or to the attribute's default value otherwise.
If Desc has a [[Get]] field and SameValue(Desc.[[Get]], current.[[Get]]) is
false, return false.
If Desc has a [[Set]] field and SameValue(Desc.[[Set]], current.[[Set]]) is
false, return false.
Else if current.[[Writable]] is false,
then
If Desc has a [[Writable]] field and
Desc.[[Writable]] is true, return
false.
NOTE: SameValue returns true for
NaN values which may be distinguishable by other means. Returning here
ensures that any existing property of O remains unmodified.
If Desc has a [[Value]] field, return SameValue(Desc.[[Value]], current.[[Value]]).
If Desc has a [[Configurable]] field, let
configurable be Desc.[[Configurable]]; else let
configurable be current.[[Configurable]].
If Desc has a [[Enumerable]] field, let
enumerable be Desc.[[Enumerable]]; else let
enumerable be current.[[Enumerable]].
Replace the property named P of object O with an accessor
property whose [[Configurable]] and [[Enumerable]] attributes are set to configurable and
enumerable, respectively, and whose [[Get]] and [[Set]] attributes are set to the value of the corresponding field in
Desc if Desc has that field, or to the attribute's default value otherwise.
If Desc has a [[Configurable]] field, let
configurable be Desc.[[Configurable]]; else let
configurable be current.[[Configurable]].
If Desc has a [[Enumerable]] field, let
enumerable be Desc.[[Enumerable]]; else let
enumerable be current.[[Enumerable]].
Replace the property named P of object O with a data
property whose [[Configurable]] and [[Enumerable]] attributes are set to configurable and
enumerable, respectively, and whose [[Value]] and [[Writable]] attributes are set to the value of the corresponding field
in Desc if Desc has that field, or to the attribute's default value otherwise.
Else,
For each field of Desc, set the corresponding attribute of the property named
P of object O to the value of the field.
The abstract operation OrdinaryOwnPropertyKeys takes argument O (an Object) and returns a
List of property keys. It performs the following steps
when called:
For each own property
keyP of O such that P is an array index, in ascending
numeric index order, do
Append P to keys.
For each own property
keyP of O such that Pis a String and P is
not an array index,
in ascending chronological order of property creation, do
Append P to keys.
For each own property
keyP of O such that Pis a Symbol, in ascending
chronological order of property creation, do
Append P to keys.
Return keys.
10.1.12 OrdinaryObjectCreate ( proto [ ,
additionalInternalSlotsList ] )
The abstract operation OrdinaryObjectCreate takes argument proto (an Object or
null) and optional argument additionalInternalSlotsList (a List of names of internal slots) and
returns an Object. It is used to specify the runtime creation of new ordinary objects.
additionalInternalSlotsList contains the names of additional internal slots that must be defined
as part of the object, beyond [[Prototype]] and [[Extensible]]. If additionalInternalSlotsList is not provided, a new empty
List is used. It performs the following
steps when called:
Let internalSlotsList be « [[Prototype]], [[Extensible]] ».
If additionalInternalSlotsList is present, set internalSlotsList to the
list-concatenation of internalSlotsList and
additionalInternalSlotsList.
Although OrdinaryObjectCreate does little more than call MakeBasicObject, its use communicates the
intention to create an ordinary object, and not an exotic one. Thus, within this
specification, it is not called by any algorithm that subsequently modifies the internal methods of the
object in ways that would make the result non-ordinary. Operations that create exotic objects invoke
MakeBasicObject directly.
The abstract operation OrdinaryCreateFromConstructor takes arguments constructor (a function object) and
intrinsicDefaultProto (a String) and optional argument internalSlotsList (a List of names of internal slots) and
returns either a normal completion containing an Object
or a throw completion. It creates an
ordinary object
whose [[Prototype]] value is retrieved from a constructor's "prototype"
property, if it exists. Otherwise the intrinsic named by intrinsicDefaultProto is used for [[Prototype]]. internalSlotsList contains the names of additional internal
slots that must be defined as part of the object. If internalSlotsList is not provided, a new
empty List is used. It performs the following
steps when called:
Assert:
intrinsicDefaultProto is this specification's name of an intrinsic object. The corresponding
object must be an intrinsic that is intended to be used as the [[Prototype]]
value of an object.
The abstract operation GetPrototypeFromConstructor takes arguments constructor (a function object) and
intrinsicDefaultProto (a String) and returns either a normal completion containing an Object
or a throw completion. It determines the [[Prototype]] value that should be used to create an object corresponding to a
specific constructor. The
value is retrieved from the constructor's "prototype" property, if it exists.
Otherwise the intrinsic named by intrinsicDefaultProto is used for [[Prototype]]. It performs the following steps when called:
Assert:
intrinsicDefaultProto is this specification's name of an intrinsic object. The corresponding
object must be an intrinsic that is intended to be used as the [[Prototype]]
value of an object.
Set proto to realm's intrinsic object named
intrinsicDefaultProto.
Return proto.
Note
If constructor does not supply a [[Prototype]] value, the default
value that is used is obtained from the realm of the constructor function rather than from the
running
execution context.
10.1.15 RequireInternalSlot ( O, internalSlot )
The abstract operation RequireInternalSlot takes arguments O (an ECMAScript
language value) and internalSlot (an internal slot name) and returns either a
normal completion containingunused or a throw completion. It
throws an exception unless Ois an Object and has the given internal slot. It performs the
following steps when called:
The PrivateEnvironment Record for Private Names
that the function was closed over. null if this function is not syntactically
contained within a class. Used as the outer PrivateEnvironment for inner classes when evaluating the
code of the function.
The script or module in which the function was created.
[[ThisMode]]
lexical, strict, or global
Defines how this references are interpreted within the formal parameters and code body
of the function. lexical means that this refers to the
this value of a lexically enclosing function. strict means
that the this value is used exactly as provided by an invocation of the function.
global means that a this value of
undefined or null is interpreted as a reference to the
global
object, and any other this value is first passed to ToObject.
If the function is created as the initializer of a class field, the name to use for NamedEvaluation of the field;
empty otherwise.
[[IsClassConstructor]]
a Boolean
Indicates whether the function is a class constructor. (If true, invoking the
function's [[Call]] will immediately throw a TypeError
exception.)
All ECMAScript function
objects have the [[Call]] internal method defined here. ECMAScript
functions that are also constructors in addition have the [[Construct]] internal method.
When calleeContext is removed from the execution context stack in step
7 it must not be destroyed if it is suspended and
retained for later resumption by an accessible Generator.
10.2.1.1 PrepareForOrdinaryCall ( F, newTarget )
The abstract operation PrepareForOrdinaryCall takes arguments F (an ECMAScript function object) and
newTarget (an Object or undefined) and returns an execution
context. It performs the following steps when called:
The abstract operation OrdinaryCallBindThis takes arguments F (an ECMAScript function object),
calleeContext (an execution context), and thisArgument (an
ECMAScript language value) and returns
unused. It performs the following steps when called:
Let thisMode be F.[[ThisMode]].
If thisMode is lexical, return unused.
Let calleeRealm be F.[[Realm]].
Let localEnv be the LexicalEnvironment of calleeContext.
Even though field initializers constitute a function boundary, calling FunctionDeclarationInstantiation does
not have any observable effect and so is omitted.
The abstract operation OrdinaryFunctionCreate takes arguments functionPrototype (an Object),
sourceText (a sequence of Unicode code points), ParameterList (a Parse Node),
Body (a Parse Node), thisMode
(lexical-this or non-lexical-this), env (an
Environment
Record), and privateEnv (a PrivateEnvironment Record or
null) and returns an ECMAScript function object. It is used to specify the runtime creation of a
new function with a default [[Call]] internal method and no [[Construct]] internal method (although one may be subsequently added by an operation
such as MakeConstructor). sourceText is the source text of
the syntactic definition of the function to be created. It performs the following steps when called:
Let internalSlotsList be the internal slots listed in Table 28.
The abstract operation AddRestrictedFunctionProperties takes arguments F (a function object) and
realm (a Realm
Record) and returns unused. It performs the following steps when
called:
The abstract operation MakeConstructor takes argument F (an ECMAScript function object or a
built-in function
object) and optional arguments writablePrototype (a Boolean) and
prototype (an Object) and returns unused. It converts F into a
constructor. It performs
the following steps when called:
The abstract operation MakeClassConstructor takes argument F (an ECMAScript function object) and
returns unused. It performs the following steps when called:
The abstract operation MakeMethod takes arguments F (an ECMAScript function object) and
homeObject (an Object) and returns unused. It configures F as a
method. It performs the following steps when called:
The abstract operation SetFunctionName takes arguments F (a function object) and name (a property key or Private Name) and
optional argument prefix (a String) and returns unused. It adds a
"name" property to F. It performs the following steps when called:
Assert: F is an
extensible object that does not have a "name" own property.
The abstract operation SetFunctionLength takes arguments F (a function object) and length (a
non-negative integer or +∞) and
returns unused. It adds a "length" property to F. It
performs the following steps when called:
Assert: F is an
extensible object that does not have a "length" own property.
When an execution context is established for evaluating an
ECMAScript function a new Function Environment Record is created and
bindings for each formal parameter are instantiated in that Environment Record. Each declaration
in the function body is also instantiated. If the function's formal parameters do not include any
default value initializers then the body declarations are instantiated in the same Environment
Record as the parameters. If default value parameter initializers exist, a second
Environment
Record is created for the body declarations. Formal parameters and functions are
initialized as part of FunctionDeclarationInstantiation. All other bindings are initialized during
evaluation of the function body.
Let fn be the sole element of the BoundNames of d.
If functionNames does not contain fn, then
Insert fn as the first element of functionNames.
NOTE: If there are multiple function declarations for the same name, the last declaration
is used.
Insert d as the first element of functionsToInitialize.
Let argumentsObjectNeeded be true.
If func.[[ThisMode]] is lexical, then
NOTE: Arrow functions never have an arguments object.
Set argumentsObjectNeeded to false.
Else if parameterNames contains "arguments", then
Set argumentsObjectNeeded to false.
Else if hasParameterExpressions is false, then
If functionNames contains "arguments" or lexicalNames
contains "arguments", then
Set argumentsObjectNeeded to false.
If strict is true or hasParameterExpressions is
false, then
NOTE: Only a single Environment Record is needed for the parameters,
since calls to eval in strict mode code cannot create new bindings which
are visible outside of the eval.
Let env be the LexicalEnvironment of calleeContext.
Else,
NOTE: A separate Environment Record is needed to ensure that
bindings created by direct
eval calls in the formal parameter list are outside the environment where
parameters are declared.
Let calleeEnv be the LexicalEnvironment of calleeContext.
Set the LexicalEnvironment of calleeContext to env.
For each String paramName of parameterNames, do
Let alreadyDeclared be ! env.HasBinding(paramName).
NOTE: Early
errors ensure that duplicate parameter names can only occur in non-strict
functions that do not have parameter default values or rest parameters.
NOTE: A mapped argument object is only provided for non-strict functions that don't
have a rest parameter, any parameter default value initializers, or any destructured parameters.
NOTE: The following step cannot return a ReturnCompletion because the only way such a completion
can arise in expression position is by use of YieldExpression, which is forbidden in parameter lists by
Early Error rules in 15.5.1 and
15.6.1.
NOTE: Only a single Environment Record is needed for the parameters
and top-level vars.
Let instantiatedVarNames be a copy of the ListparameterBindings.
For each element n of varNames, do
If instantiatedVarNames does not contain n, then
Append n to instantiatedVarNames.
Perform ! env.CreateMutableBinding(n,
false).
Perform ! env.InitializeBinding(n,
undefined).
Let varEnv be env.
Else,
NOTE: A separate Environment Record is needed to ensure that
closures created by expressions in the formal parameter list do not have visibility of declarations
in the function body.
NOTE: Non-strict functions use a separate Environment
Record for top-level lexical declarations so that a direct eval can determine
whether any var scoped declarations introduced by the eval code conflict with pre-existing top-level
lexically scoped declarations. This is not needed for strict functions because a strict
direct eval always places
all declarations into a new Environment Record.
Else,
Let lexEnv be varEnv.
Set the LexicalEnvironment of calleeContext to lexEnv.
NOTE: A lexically declared name cannot be the same as a function/generator declaration, formal
parameter, or a var name. Lexically declared names are only instantiated here but not initialized.
B.3.2
provides an extension to the above algorithm that is necessary for backwards compatibility with web
browser implementations of ECMAScript that predate ECMAScript 2015.
In addition to the internal slots required of every ordinary object (see 10.1), a built-in function object must also
have the following internal slots:
[[Realm]], a Realm Record that represents the realm in which the function was created.
[[InitialName]], a String that is the initial name of the function. It is used by
20.2.3.5.
A built-in function
object must have a [[Call]] internal method that conforms to the
definition in 10.3.1.
A built-in function
object has a [[Construct]] internal method if and only if it is
described as a “constructor”, or some algorithm in this specification explicitly sets
its [[Construct]] internal method. Such a [[Construct]]
internal method must conform to the definition in 10.3.2.
An implementation may provide additional built-in function objects that are not defined in this specification.
Let result be the Completion Record that is the result of evaluatingF in a manner that conforms to the
specification of F. If thisArgument is uninitialized, the
this value is uninitialized; otherwise thisArgument provides the
this value. argumentsList provides the named parameters.
newTarget provides the NewTarget value.
NOTE: If F is defined in this document, “the specification of F” is the
behaviour specified for it via algorithm steps or other means.
When calleeContext is removed from the execution context stack it must not
be destroyed if it has been suspended and retained by an accessible Generator for later resumption.
The abstract operation CreateBuiltinFunction takes arguments behaviour (an Abstract
Closure, a set of algorithm steps, or some other definition of a function's behaviour
provided in this specification), length (a non-negative integer or +∞), name (a property key or a Private Name), and
additionalInternalSlotsList (a List of names of internal
slots) and optional arguments realm (a Realm Record), prototype (an Object or
null), and prefix (a String) and returns a built-in function object.
additionalInternalSlotsList contains the names of additional internal slots that must be defined
as part of the object. This operation creates a built-in function object. It performs the following steps when called:
If prototype is not present, set prototype to realm.[[Intrinsics]].[[%Function.prototype%]].
Let internalSlotsList be a List containing the
names of all the internal slots that 10.3 requires for the built-in function object that
is about to be created.
Append to internalSlotsList the elements of additionalInternalSlotsList.
Let func be a new built-in function object that, when called, performs the action
described by behaviour using the provided arguments as the values of the corresponding
parameters specified by behaviour. The new function object has internal slots whose
names are the elements of internalSlotsList, and an [[InitialName]]
internal slot.
Each built-in function defined in this specification is created by calling the CreateBuiltinFunction
abstract operation.
10.4 Built-in Exotic Object Internal Methods and Slots
This specification defines several kinds of built-in exotic objects. These objects generally behave similar to ordinary objects except for
a few specific situations. The following exotic objects use the ordinary object internal methods except where it is
explicitly specified otherwise below:
An object is a bound function exotic object if its [[Call]] and (if
applicable) [[Construct]] internal methods use the following implementations, and
its other essential internal methods use the definitions found in 10.1. These methods are
installed in BoundFunctionCreate.
Set obj.[[BoundTargetFunction]] to targetFunction.
Set obj.[[BoundThis]] to boundThis.
Set obj.[[BoundArguments]] to boundArgs.
Return obj.
10.4.2 Array Exotic Objects
An Array is an exotic
object that gives special treatment to array indexproperty keys (see 6.1.7). A property whose property name is an array index is also called an
element. Every Array has a non-configurable "length" property whose value is
always a non-negative integral
Number whose mathematical value is strictly less than 232. The
value of the "length" property is numerically greater than the name of every own property
whose name is an array
index; whenever an own property of an Array is created or changed, other properties are
adjusted as necessary to maintain this invariant. Specifically, whenever an own property is added whose name
is an array index, the
value of the "length" property is changed, if necessary, to be one more than the numeric
value of that array
index; and whenever the value of the "length" property is changed,
every own property whose name is an array index whose value is not smaller than the new length is
deleted. This constraint applies only to own properties of an Array and is unaffected by
"length" or array
index properties that may be inherited from its prototypes.
An object is an Array exotic
object (or simply, an Array) if its [[DefineOwnProperty]] internal method
uses the following implementation, and its other essential internal methods use the definitions found in
10.1. These methods are
installed in ArrayCreate.
The abstract operation ArrayCreate takes argument length (a non-negative integer) and optional argument
proto (an Object) and returns either a normal completion containing an
Array exotic
object or a throw completion. It
is used to specify the creation of new Arrays. It performs the following steps when called:
If length > 232 - 1, throw a RangeError exception.
The abstract operation ArraySpeciesCreate takes arguments originalArray (an Object) and
length (a non-negative integer) and returns either a normal completion containing an Object
or a throw completion. It is used to
specify the creation of a new Array or similar object using a constructor function that is derived from
originalArray. It does not enforce that the constructor function returns an Array. It performs the following
steps when called:
If originalArray was created using the standard built-in Array constructor for a realm that is not the realm of the running
execution context, then a new Array is created using the realm of the running
execution context. This maintains compatibility with Web browsers that have
historically had that behaviour for the Array.prototype methods that now are defined
using ArraySpeciesCreate.
In steps 3 and 4, if Desc.[[Value]] is an object then its valueOf method is called twice.
This is legacy behaviour that was specified with this effect starting with the 2nd Edition
of this specification.
10.4.3 String Exotic Objects
A String object is an exotic
object that encapsulates a String value and exposes virtual integer-indexeddata properties
corresponding to the individual code unit elements of the String value. String exotic
objects always have a data property named "length" whose value is
the length of the encapsulated String value. Both the code unit data properties and the
"length" property are non-writable and non-configurable.
An object is a String exotic
object (or simply, a String object) if its [[GetOwnProperty]], [[DefineOwnProperty]], and [[OwnPropertyKeys]] internal
methods use the following implementations, and its other essential internal methods use the definitions
found in 10.1. These methods are
installed in StringCreate.
For each own property
keyP of O such that Pis a String and P is
not an array index,
in ascending chronological order of property creation, do
Append P to keys.
For each own property
keyP of O such that Pis a Symbol, in ascending
chronological order of property creation, do
Append P to keys.
Return keys.
10.4.3.4 StringCreate ( value, prototype )
The abstract operation StringCreate takes arguments value (a String) and prototype
(an Object) and returns a String exotic object. It is used to specify the creation
of new String exotic
objects. It performs the following steps when called:
Let S be MakeBasicObject(« [[Prototype]], [[Extensible]], [[StringData]] »).
Set S.[[Prototype]] to prototype.
Set S.[[StringData]] to value.
Set S.[[GetOwnProperty]] as specified in 10.4.3.1.
Set S.[[DefineOwnProperty]] as specified in 10.4.3.2.
Set S.[[OwnPropertyKeys]] as specified in 10.4.3.3.
The abstract operation StringGetOwnProperty takes arguments S (an Object that has a [[StringData]] internal slot) and P (a property key) and returns a Property Descriptor or
undefined. It performs the following steps when called:
Most ECMAScript functions make an arguments object available to their code. Depending upon the
characteristics of the function definition, its arguments object is either an ordinary object or an
arguments exotic
object. An arguments exotic object is an exotic object whose array index properties map to the
formal parameters bindings of an invocation of its associated ECMAScript function.
An object is an arguments exotic object if its internal methods use the following implementations,
with the ones not specified here using those found in 10.1. These methods are
installed in CreateMappedArgumentsObject.
Arguments
exotic objects have the same internal slots as ordinary objects. They also have a [[ParameterMap]] internal slot. Ordinary arguments objects also have a [[ParameterMap]] internal slot whose value is always undefined. For
ordinary argument objects the [[ParameterMap]] internal slot is only used by
Object.prototype.toString (20.1.3.6) to identify them as such.
Note 2
The integer-indexeddata properties of an arguments exotic
object whose numeric name values are less than the number of formal parameters of the
corresponding function
object initially share their values with the corresponding argument bindings in the
function's execution context. This means that changing the
property changes the corresponding value of the argument binding and vice-versa. This correspondence is
broken if such a property is deleted and then redefined or if the property is changed into an accessor property. If
the arguments object is an ordinary object, the values of its properties are simply a
copy of the arguments passed to the function and there is no dynamic linkage between the property values
and the formal parameter values.
Note 3
The ParameterMap object and its property values are used as a device for specifying the arguments
object correspondence to argument bindings. The ParameterMap object and the objects that are the values
of its properties are not directly observable from ECMAScript code. An ECMAScript implementation does
not need to actually create or use such objects to implement the specified semantics.
Note 4
Ordinary arguments objects define a non-configurable accessor property named
"callee" which throws a TypeError exception on access. The
"callee" property has a more specific meaning for arguments exotic
objects, which are created only for some class of non-strict
functions. The definition of this property in the ordinary variant exists to ensure
that it is not defined in any other manner by conforming ECMAScript implementations.
Note 5
ECMAScript implementations of arguments exotic objects have historically contained
an accessor
property named "caller". Prior to ECMAScript 2017, this
specification included the definition of a throwing "caller" property on ordinary
arguments objects. Since implementations do not contain this extension any longer, ECMAScript 2017
dropped the requirement for a throwing "caller" accessor.
The abstract operation CreateUnmappedArgumentsObject takes argument argumentsList (a List of ECMAScript
language values) and returns an ordinary object. It performs the following steps when called:
Let len be the number of elements in argumentsList.
The abstract operation MakeArgGetter takes arguments name (a String) and env (an
Environment
Record) and returns a function object. It creates a built-in function object that
when executed returns the value bound for name in env. It performs the following
steps when called:
Let getterClosure be a new Abstract Closure with no parameters that captures
name and env and performs the following steps when called:
NOTE: getter is never directly accessible to ECMAScript code.
Return getter.
10.4.4.7.2 MakeArgSetter ( name, env )
The abstract operation MakeArgSetter takes arguments name (a String) and env (an
Environment
Record) and returns a function object. It creates a built-in function object that
when executed sets the value bound for name in env. It performs the following
steps when called:
Let setterClosure be a new Abstract Closure with parameters (value)
that captures name and env and performs the following steps when called:
NOTE: setter is never directly accessible to ECMAScript code.
Return setter.
10.4.5 TypedArray Exotic Objects
A TypedArray is an
exotic object that
performs special handling of property
keys that are canonical numeric strings, using the subset that
are in-bounds integer
indices to index elements of uniform type and enforcing the invariant that the remainder
are absent without incurring prototype chain traversal.
TypedArrays have the same
internal slots as ordinary
objects and additionally [[ViewedArrayBuffer]], [[TypedArrayName]], [[ContentType]], [[ByteLength]], [[ByteOffset]], and [[ArrayLength]] internal slots.
An object is a TypedArray if its [[PreventExtensions]], [[GetOwnProperty]], [[HasProperty]], [[DefineOwnProperty]], [[Get]], [[Set]], [[Delete]], and
[[OwnPropertyKeys]], internal methods use the definitions in this section, and its
other essential internal methods use the definitions found in 10.1. These methods are
installed by TypedArrayCreate.
10.4.5.1[[PreventExtensions]] ( )
The [[PreventExtensions]] internal method of a TypedArrayO takes no arguments and
returns a normal completion containing a
Boolean. It performs the following steps when called:
NOTE: The extensibility-related invariants specified in 6.1.7.3 do not allow this
method to return true when O can gain (or lose and then regain)
properties, which might occur for properties with integer index names when its underlying buffer is resized.
For each own property
keyP of O such that Pis a Symbol, in ascending
chronological order of property creation, do
Append P to keys.
Return keys.
10.4.5.9 TypedArray With Buffer Witness Records
An TypedArray With Buffer Witness
Record is a Record value used to encapsulate a
TypedArray along with a
cached byte length of the viewed buffer. It is used to help ensure there is a single shared memory read
event of the byte length data block when the viewed buffer is a growable
SharedArrayBuffer.
TypedArray With Buffer Witness Records have the fields listed in Table 30.
The byte length of the object's [[ViewedArrayBuffer]] when the
Record was created.
10.4.5.10 MakeTypedArrayWithBufferWitnessRecord ( obj,
order )
The abstract operation MakeTypedArrayWithBufferWitnessRecord takes arguments obj (a TypedArray) and order
(seq-cst or unordered) and returns a TypedArray With Buffer Witness
Record. It performs the following steps when called:
The abstract operation TypedArrayCreate takes argument prototype (an Object) and returns a
TypedArray. It is used to
specify the creation of new TypedArrays. It performs the following steps when called:
Let internalSlotsList be « [[Prototype]], [[Extensible]], [[ViewedArrayBuffer]], [[TypedArrayName]], [[ContentType]], [[ByteLength]], [[ByteOffset]], [[ArrayLength]] ».
Set A.[[OwnPropertyKeys]] as specified in 10.4.5.8.
Set A.[[Prototype]] to prototype.
Return A.
10.4.5.12 TypedArrayByteLength ( taRecord )
The abstract operation TypedArrayByteLength takes argument taRecord (a TypedArray With Buffer Witness
Record) and returns a non-negative integer. It performs the following steps when called:
The abstract operation TypedArrayLength takes argument taRecord (a TypedArray With Buffer Witness
Record) and returns a non-negative integer. It performs the following steps when called:
The abstract operation IsTypedArrayOutOfBounds takes argument taRecord (a TypedArray With Buffer Witness
Record) and returns a Boolean. It checks if any of the object's numeric properties
reference a value at an index not contained within the underlying buffer's bounds. It performs the
following steps when called:
Let O be taRecord.[[Object]].
Let bufferByteLength be taRecord.[[CachedBufferByteLength]].
Assert: IsDetachedBuffer(O.[[ViewedArrayBuffer]]) is true if and only if
bufferByteLength is detached.
The abstract operation IsValidIntegerIndex takes arguments O (a TypedArray) and index (a Number) and
returns a Boolean. It performs the following steps when called:
If IsDetachedBuffer(O.[[ViewedArrayBuffer]]) is true, return
false.
The abstract operation TypedArrayGetElement takes arguments O (a TypedArray) and index (a Number) and
returns a Number, a BigInt, or undefined. It performs the following steps when called:
This operation always appears to succeed, but it has no effect when attempting to write past the end
of a TypedArray or to
a TypedArray which is
backed by a detached ArrayBuffer.
10.4.5.19 IsArrayBufferViewOutOfBounds ( O )
The abstract operation IsArrayBufferViewOutOfBounds takes argument O (a TypedArray or a DataView) and
returns a Boolean. It checks if either any of a TypedArray's numeric properties or a DataView object's methods can
reference a value at an index not contained within the underlying data block's bounds. This abstract
operation exists as a convenience for upstream specifications. It performs the following steps when
called:
A module namespace exotic object is an exotic object that exposes
the bindings exported from an ECMAScript Module
(See 16.2.3). There is a
one-to-one correspondence between the String-keyed own properties of a module
namespace exotic object and the binding names exported by the Module. The exported bindings include any bindings that are indirectly
exported using export * export items. Each String-valued own property key is the StringValue of the corresponding
exported binding name. These are the only String-keyed properties of a module
namespace exotic object. Each such property has the attributes { [[Writable]]: true, [[Enumerable]]:
true, [[Configurable]]: false }. Module
namespace exotic objects are not extensible.
An object is a module namespace exotic object if its [[GetPrototypeOf]],
[[SetPrototypeOf]], [[IsExtensible]], [[PreventExtensions]], [[GetOwnProperty]], [[DefineOwnProperty]], [[HasProperty]], [[Get]], [[Set]], [[Delete]], and
[[OwnPropertyKeys]] internal methods use the definitions in this section, and its
other essential internal methods use the definitions found in 10.1. These methods are
installed by ModuleNamespaceCreate.
A List whose elements are the
String values of the exported names exposed as own properties of this object. The list is sorted
according to lexicographic code unit order.
ResolveExport is side-effect free. Each time this operation is called with a specific
exportName, resolveSet pair as arguments it must return the same result. An
implementation might choose to pre-compute or cache the ResolveExport results for the [[Exports]] of each module namespace exotic
object.
An object is an immutable prototype exotic object if its [[SetPrototypeOf]]
internal method uses the following implementation. (Its other essential internal methods may use any
implementation, depending on the specific immutable prototype exotic
object in question.)
The abstract operation SetImmutablePrototype takes arguments O (an Object) and V
(an Object or null) and returns either a normal completion containing a Boolean
or a throw completion. It performs the
following steps when called:
10.5 Proxy Object Internal Methods and Internal Slots
A Proxy object is an exotic
object whose essential internal methods are partially implemented using ECMAScript code.
Every Proxy object has an internal slot called [[ProxyHandler]]. The value of [[ProxyHandler]] is an object, called the proxy's handler object, or
null. Methods (see Table 32) of a handler object may be used to augment
the implementation for one or more of the Proxy object's internal methods. Every Proxy object also has an
internal slot called [[ProxyTarget]] whose value is either an object or
null. This object is called the proxy's target object.
An object is a Proxy exotic
object if its essential internal methods (including [[Call]] and [[Construct]], if applicable) use the definitions in this section. These internal
methods are installed in ProxyCreate.
Table 32: Proxy Handler Methods
Internal Method
Handler Method
[[GetPrototypeOf]]
getPrototypeOf
[[SetPrototypeOf]]
setPrototypeOf
[[IsExtensible]]
isExtensible
[[PreventExtensions]]
preventExtensions
[[GetOwnProperty]]
getOwnPropertyDescriptor
[[DefineOwnProperty]]
defineProperty
[[HasProperty]]
has
[[Get]]
get
[[Set]]
set
[[Delete]]
deleteProperty
[[OwnPropertyKeys]]
ownKeys
[[Call]]
apply
[[Construct]]
construct
When a handler method is called to provide the implementation of a Proxy object internal method, the handler
method is passed the proxy's target object as a parameter. A proxy's handler object does not necessarily have
a method corresponding to every essential internal method. Invoking an internal method on the proxy results in
the invocation of the corresponding internal method on the proxy's target object if the handler object does
not have a method corresponding to the internal trap.
The [[ProxyHandler]] and [[ProxyTarget]] internal slots of
a Proxy object are always initialized when the object is created and typically may not be modified. Some Proxy
objects are created in a manner that permits them to be subsequently revoked. When a proxy is
revoked, its [[ProxyHandler]] and [[ProxyTarget]] internal
slots are set to null causing subsequent invocations of internal methods on that Proxy
object to throw a TypeError exception.
Because Proxy objects permit the implementation of internal methods to be provided by arbitrary ECMAScript
code, it is possible to define a Proxy object whose handler methods violates the invariants defined in
6.1.7.3. Some of the internal
method invariants defined in 6.1.7.3 are
essential integrity invariants. These invariants are explicitly enforced by the Proxy object internal methods
specified in this section. An ECMAScript implementation must be robust in the presence of all possible
invariant violations.
If SameValue(handlerProto, targetProto) is
false, throw a TypeError exception.
Return handlerProto.
Note
[[GetPrototypeOf]] for Proxy objects enforces the following invariants:
The result of [[GetPrototypeOf]] must be either an Object or
null.
If the target object is not extensible, [[GetPrototypeOf]] applied to the
Proxy object must return the same value as [[GetPrototypeOf]] applied to the
Proxy object's target object.
[[IsExtensible]] applied to the Proxy object must return the same value as
[[IsExtensible]] applied to the Proxy object's target object with the same
argument.
If targetDesc.[[Writable]] is true,
throw a TypeError exception.
Return resultDesc.
Note
[[GetOwnProperty]] for Proxy objects enforces the following invariants:
The result of [[GetOwnProperty]] must be either an Object or
undefined.
A property cannot be reported as non-existent, if it exists as a non-configurable own property of the
target object.
A property cannot be reported as non-existent, if it exists as an own property of a non-extensible
target object.
A property cannot be reported as existent, if it does not exist as an own property of the target
object and the target object is not extensible.
A property cannot be reported as non-configurable, unless it exists as a non-configurable own property
of the target object.
A property cannot be reported as both non-configurable and non-writable, unless it exists as a
non-configurable, non-writable own property of the target object.
If settingConfigFalse is true and targetDesc.[[Configurable]] is true, throw a
TypeError exception.
If IsDataDescriptor(targetDesc) is
true, targetDesc.[[Configurable]] is
false, and targetDesc.[[Writable]] is
true, then
If Desc has a [[Writable]] field and Desc.[[Writable]] is false, throw a
TypeError exception.
Return true.
Note
[[DefineOwnProperty]] for Proxy objects enforces the following invariants:
The result of [[DefineOwnProperty]]is a Boolean value.
A property cannot be added, if the target object is not extensible.
A property cannot be non-configurable, unless there exists a corresponding non-configurable own
property of the target object.
A non-configurable property cannot be non-writable, unless there exists a corresponding
non-configurable, non-writable own property of the target object.
If a property has a corresponding target object property then applying the Property Descriptor of the
property to the target object using [[DefineOwnProperty]] will not throw an
exception.
Let trapResult be ? Call(trap, handler, « target,
P, Receiver »).
Let targetDesc be ? target.[[GetOwnProperty]](P).
If targetDesc is not undefined and targetDesc.[[Configurable]] is false, then
If IsDataDescriptor(targetDesc) is
true and targetDesc.[[Writable]] is
false, then
If SameValue(trapResult,
targetDesc.[[Value]]) is false, throw a
TypeError exception.
If IsAccessorDescriptor(targetDesc) is
true and targetDesc.[[Get]] is
undefined, then
If trapResult is not undefined, throw a
TypeError exception.
Return trapResult.
Note
[[Get]] for Proxy objects enforces the following invariants:
The value reported for a property must be the same as the value of the corresponding target object
property if the target object property is a non-writable, non-configurable own data property.
The value reported for a property must be undefined if the corresponding target
object property is a non-configurable own accessor property that has undefined as
its [[Get]] attribute.
Cannot change the value of a property to be different from the value of the corresponding target
object property if the corresponding target object property is a non-writable, non-configurable own
data
property.
Cannot set the value of a property if the corresponding target object property is a non-configurable
own accessor
property that has undefined as its [[Set]] attribute.
A Proxy exotic
object only has a [[Call]] internal method if the initial
value of its [[ProxyTarget]] internal slot is an object that has a [[Call]] internal method.
A Proxy exotic
object only has a [[Construct]] internal method if the
initial value of its [[ProxyTarget]] internal slot is an object that has a [[Construct]] internal method.
Note 2
[[Construct]] for Proxy objects enforces the following invariants:
The result of [[Construct]] must be an Object.
10.5.14 ValidateNonRevokedProxy ( proxy )
The abstract operation ValidateNonRevokedProxy takes argument proxy (a Proxy exotic
object) and returns either a normal completion
containingunused or a throw completion. It throws a
TypeError exception if proxy has been revoked. It performs the following steps
when called:
If proxy.[[ProxyTarget]] is null, throw a
TypeError exception.
ECMAScript source text is a sequence of Unicode code points. All Unicode code point
values from U+0000 to U+10FFFF, including surrogate code points, may occur in ECMAScript source text where
permitted by the ECMAScript grammars. The actual encodings used to store and interchange ECMAScript source
text is not relevant to this specification. Regardless of the external source text encoding, a conforming
ECMAScript implementation processes the source text as if it was an equivalent sequence of SourceCharacter values, each SourceCharacter being a Unicode code point.
Conforming ECMAScript implementations are not required to perform any normalization of source text, or behave
as though they were performing normalization of source text.
The components of a combining character sequence are treated as individual Unicode code points even though a
user might think of the whole sequence as a single character.
Note
In string literals, regular expression literals, template literals and identifiers, any Unicode code
point may also be expressed using Unicode escape sequences that explicitly express a code point's numeric
value. Within a comment, such an escape sequence is effectively ignored as part of the comment.
ECMAScript differs from the Java programming language in the behaviour of Unicode escape sequences. In a
Java program, if the Unicode escape sequence \u000A, for example, occurs within a single-line
comment, it is interpreted as a line terminator (Unicode code point U+000A is LINE FEED (LF)) and
therefore the next code point is not part of the comment. Similarly, if the Unicode escape sequence
\u000A occurs within a string literal in a Java program, it is likewise interpreted as a line
terminator, which is not allowed within a string literal—one must write \n instead of
\u000A to cause a LINE FEED (LF) to be part of the value of a string literal. In an
ECMAScript program, a Unicode escape sequence occurring within a comment is never interpreted and
therefore cannot contribute to termination of the comment. Similarly, a Unicode escape sequence occurring
within a string literal in an ECMAScript program always contributes to the literal and is never
interpreted as a line terminator or as a code point that might terminate the string literal.
The abstract operation UTF16EncodeCodePoint takes argument cp (a Unicode code point) and returns
a String. It performs the following steps when called:
11.1.2 Static Semantics: CodePointsToString ( text )
The abstract operation CodePointsToString takes argument text (a sequence of Unicode code
points) and returns a String. It converts text into a String value, as described in 6.1.4. It performs the following steps
when called:
The abstract operation UTF16SurrogatePairToCodePoint takes arguments lead (a code unit) and
trail (a code unit) and returns a code point. Two code units that form a UTF-16 surrogate pair are
converted to a code point. It performs the following steps when called:
Let cp be (lead - 0xD800) × 0x400 + (trail - 0xDC00) + 0x10000.
Return the code point cp.
11.1.4 Static Semantics: CodePointAt ( string, position
)
The abstract operation CodePointAt takes arguments string (a String) and position (a
non-negative integer) and returns
a Record with fields [[CodePoint]] (a code point), [[CodeUnitCount]] (a positive
integer), and [[IsUnpairedSurrogate]] (a Boolean). It interprets string as a sequence of
UTF-16 encoded code points, as described in 6.1.4, and reads from it
a single code point starting with the code unit at index position. It performs the following
steps when called:
The abstract operation StringToCodePoints takes argument string (a String) and returns a
List of code points. It returns the
sequence of Unicode code points that results from interpreting string as UTF-16 encoded Unicode
text as described in 6.1.4. It performs the following steps
when called:
The abstract operation ParseText takes arguments sourceText (a String or a sequence of Unicode
code points) and goalSymbol (a nonterminal in one of the ECMAScript grammars) and returns a
Parse
Node or a non-empty List of
SyntaxError objects. It performs the following steps when called:
If the parse succeeded and no early errors were found, return the Parse
Node (an instance of goalSymbol) at the root of the parse tree resulting
from the parse.
Otherwise, return a List of one or more
SyntaxError objects representing the parsing errors and/or early errors. If more than
one parsing error or early
error is present, the number and ordering of error objects in the list is implementation-defined, but at least one must be
present.
Note 1
Consider a text that has an early
error at a particular point, and also a syntax error at a later point. An
implementation that does a parse pass followed by an early errors pass might report the syntax error and not proceed
to the early errors
pass. An implementation that interleaves the two activities might report the early error and not proceed
to find the syntax error. A third implementation might report both errors. All of these behaviours are
conformant.
Eval code is the source text supplied to the built-in eval function.
More precisely, if the parameter to the built-in eval function is a String, it is treated as an
ECMAScript Script. The eval code for a
particular invocation of eval is the global code portion of that Script.
then the source text matched by the
BindingIdentifier (if any) of that
declaration or expression is also included in the function code of the corresponding function.
Function code is generally provided as the bodies of Function Definitions (15.2),
Arrow Function Definitions (15.3), Method Definitions (15.4),
Generator Function Definitions (15.5), Async Function Definitions (15.8), Async Generator Function Definitions
(15.6), and Async Arrow Functions
(15.9). Function code is also derived from
the arguments to the Function constructor (20.2.1.1), the GeneratorFunction
constructor (27.3.1.1),
the AsyncFunction constructor (27.7.1.1), and the
AsyncGeneratorFunction constructor (27.4.1.1).
Note 2
The practical effect of including the BindingIdentifier in function code is that the Early
Errors for strict
mode code are applied to a BindingIdentifier that is the name of a function whose
body contains a "use strict" directive, even if the surrounding code is not strict mode
code.
11.2.1 Directive Prologues and the Use Strict Directive
An ECMAScript syntactic unit may be processed using either unrestricted or strict mode syntax and semantics
(4.3.2). Code is interpreted as strict mode code in the following situations:
An ECMAScript implementation may support the evaluation of function exotic objects whose evaluative behaviour is
expressed in some host-defined form of executable code other than ECMAScript source text.
Whether a function
object is defined within ECMAScript code or is a built-in function is not observable from
the perspective of ECMAScript code that calls or is called by such a function object.
12 ECMAScript Language: Lexical Grammar
The source text of an ECMAScript Script or Module is first converted into a sequence of input
elements, which are tokens, line terminators, comments, or white space. The source text is scanned from left to
right, repeatedly taking the longest possible sequence of code points as the next input element.
The use of multiple lexical goals ensures that there are no lexical ambiguities that would affect automatic
semicolon insertion. For example, there are no syntactic grammar contexts where both a leading division or
division-assignment, and a leading RegularExpressionLiteral are permitted. This is not
affected by semicolon insertion (see 12.10); in examples such as the following:
a = b
/hi/g.exec(c).map(d);
where the first non-whitespace, non-comment code point after a LineTerminator is U+002F (SOLIDUS) and the syntactic context
allows division or division-assignment, no semicolon is inserted at the LineTerminator. That is, the above example is interpreted in
the same way as:
The Unicode format-control characters (i.e., the characters in category “Cf” in the Unicode Character
Database such as LEFT-TO-RIGHT MARK or RIGHT-TO-LEFT MARK) are control codes used to control the formatting of
a range of text in the absence of higher-level protocols for this (such as mark-up languages).
It is useful to allow format-control characters in source text to facilitate editing and display. All format
control characters may be used within comments, and within string literals, template literals, and regular
expression literals.
U+FEFF (ZERO WIDTH NO-BREAK SPACE) is a format-control character used primarily at the start of a text to
mark it as Unicode and to allow detection of the text's encoding and byte order. <ZWNBSP> characters
intended for this purpose can sometimes also appear after the start of a text, for example as a result of
concatenating files. In ECMAScript
source text <ZWNBSP> code points are treated as white space characters (see 12.2) outside of comments,
string literals, template literals, and regular expression literals.
12.2 White Space
White space code points are used to improve source text readability and to separate tokens (indivisible
lexical units) from each other, but are otherwise insignificant. White space code points may occur between any
two tokens and at the start or end of input. White space code points may occur within a StringLiteral, a RegularExpressionLiteral, a Template, or a TemplateSubstitutionTail where they are considered
significant code points forming part of a literal value. They may also occur within a Comment, but cannot appear within any other kind of
token.
The ECMAScript white space code points are listed in Table 33.
Table 33: White Space Code Points
Code Points
Name
Abbreviation
U+0009
CHARACTER TABULATION
<TAB>
U+000B
LINE TABULATION
<VT>
U+000C
FORM FEED (FF)
<FF>
U+FEFF
ZERO WIDTH NO-BREAK SPACE
<ZWNBSP>
any code point in general category “Space_Separator”
<USP>
Note 1
U+0020 (SPACE) and U+00A0 (NO-BREAK SPACE) code points are part of <USP>.
Note 2
Other than for the code points listed in Table 33, ECMAScript WhiteSpace intentionally excludes all code points that have the
Unicode “White_Space” property but which are not classified in general category “Space_Separator” (“Zs”).
Like white space code points, line terminator code points are used to improve source text readability and to
separate tokens (indivisible lexical units) from each other. However, unlike white space code points, line
terminators have some influence over the behaviour of the syntactic grammar. In general, line terminators may
occur between any two tokens, but there are a few places where they are forbidden by the syntactic grammar.
Line terminators also affect the process of automatic semicolon insertion (12.10). A line terminator cannot occur within any
token except a StringLiteral, Template, or TemplateSubstitutionTail. <LF> and <CR>
line terminators cannot occur within a StringLiteral token except as part of a LineContinuation.
Line terminators are included in the set of white space code points that are matched by the \s
class in regular expressions.
The ECMAScript line terminator code points are listed in Table 34.
Table 34: Line Terminator Code Points
Code Point
Unicode Name
Abbreviation
U+000A
LINE FEED (LF)
<LF>
U+000D
CARRIAGE RETURN (CR)
<CR>
U+2028
LINE SEPARATOR
<LS>
U+2029
PARAGRAPH SEPARATOR
<PS>
Only the Unicode code points in Table 34 are treated as line terminators. Other
new line or line breaking Unicode code points are not treated as line terminators but are treated as white
space if they meet the requirements listed in Table 33. The sequence <CR><LF> is
commonly used as a line terminator. It should be considered a single SourceCharacter for the purpose of reporting line numbers.
Comments can be either single or multi-line. Multi-line comments cannot nest.
Because a single-line comment can contain any Unicode code point except a LineTerminator code point, and because of the general rule that a
token is always as long as possible, a single-line comment always consists of all code points from the
// marker to the end of the line. However, the LineTerminator at the end of the line is not considered to be
part of the single-line comment; it is recognized separately by the lexical grammar and becomes part of the
stream of input elements for the syntactic grammar. This point is very important, because it implies that the
presence or absence of single-line comments does not affect the process of automatic semicolon insertion (see
12.10).
Comments behave like white space and are discarded except that, if a MultiLineComment contains a line terminator code point, then
the entire comment is considered to be a LineTerminator for purposes of parsing by the syntactic grammar.
IdentifierName and ReservedWord are tokens that are interpreted
according to the Default Identifier Syntax given in Unicode Standard Annex #31, Identifier and Pattern Syntax,
with some small modifications. ReservedWord
is an enumerated subset of IdentifierName.
The syntactic grammar defines Identifier as an
IdentifierName that is not a ReservedWord. The Unicode identifier grammar is
based on character properties specified by the Unicode Standard. The Unicode code points in the specified
categories in the latest version of the Unicode Standard must be treated as in those categories by all
conforming ECMAScript implementations. ECMAScript implementations may recognize identifier code points defined
in later editions of the Unicode Standard.
Note 1
This standard specifies specific code point additions: U+0024 (DOLLAR SIGN) and U+005F (LOW LINE) are
permitted anywhere in an IdentifierName.
The sets of code points with Unicode properties “ID_Start” and “ID_Continue” include, respectively, the
code points with Unicode properties “Other_ID_Start” and “Other_ID_Continue”.
12.7.1 Identifier Names
Unicode escape sequences are permitted in an IdentifierName, where they contribute a single Unicode code
point equal to the IdentifierCodePoint of the UnicodeEscapeSequence. The \ preceding the
UnicodeEscapeSequence does not
contribute any code points. A UnicodeEscapeSequence cannot be used to contribute a
code point to an IdentifierName that
would otherwise be invalid. In other words, if a \UnicodeEscapeSequence sequence were replaced by the
SourceCharacter it contributes, the
result must still be a valid IdentifierName that has the exact same sequence of SourceCharacter elements as the original
IdentifierName. All interpretations of
IdentifierName within this specification
are based upon their actual code points regardless of whether or not an escape sequence was used to
contribute any particular code point.
Two IdentifierNames that are
canonically equivalent according to the Unicode Standard are not equal unless, after replacement of
each UnicodeEscapeSequence, they
are represented by the exact same sequence of code points.
The syntax-directed operation
IdentifierCodePoints takes no arguments and returns a List of code points. It is defined
piecewise over the following productions:
Return the code point whose numeric value is the MV of CodePoint.
12.7.2 Keywords and Reserved Words
A keyword is a token that matches IdentifierName, but also has a syntactic use; that is, it
appears literally, in a fixed width font, in some syntactic production. The keywords of
ECMAScript include if, while, async, await, and many
others.
A reserved word is an IdentifierName that cannot be used as an identifier. Many
keywords are reserved words, but some are not, and some are reserved only in certain contexts.
if and while are reserved words. await is reserved only inside async
functions and modules. async is not reserved; it can be used as a variable name or statement
label without restriction.
This specification uses a combination of grammatical productions and early error rules to specify which names are valid
identifiers and which are reserved words. All tokens in the ReservedWord list below, except for await and
yield, are unconditionally reserved. Exceptions for await and yield
are specified in 13.1, using parameterized syntactic productions. Lastly, several
early error rules
restrict the set of valid identifiers. See 13.1.1, 14.3.1.1, 14.7.5.1, and
15.7.1. In summary, there are
five categories of identifier names:
Those that are always allowed as identifiers, and are not keywords, such as Math,
window, toString, and _;
Those that are never allowed as identifiers, namely the ReservedWords listed below except await and
yield;
Those that are contextually allowed as identifiers, namely await and yield;
Those that are contextually disallowed as identifiers, in strict mode code: let,
static, implements, interface, package,
private, protected, and public;
Those that are always allowed as identifiers, but also appear as keywords within certain syntactic
productions, at places where Identifier
is not allowed: as, async, from, get,
meta, of, set, and target.
The term conditional keyword, or contextual keyword, is sometimes used to refer to the
keywords that fall in the last three categories, and thus can be used as identifiers in some contexts and as
keywords in others.
Per 5.1.5, keywords in the grammar match literal sequences
of specific SourceCharacter
elements. A code point in a keyword cannot be expressed by a \UnicodeEscapeSequence.
enum is not currently used as a keyword in this specification. It is a future reserved
word, set aside for use as a keyword in future language extensions.
Similarly, implements, interface, package, private,
protected, and public are future reserved words in strict mode
code.
The syntax-directed operation
NumericValue takes no arguments and returns a Number or a BigInt. It is defined piecewise over the
following productions:
A string literal is 0 or more Unicode code points enclosed in single or double quotes. Unicode code
points may also be represented by an escape sequence. All code points may appear literally in a string
literal except for the closing quote code points, U+005C (REVERSE SOLIDUS), U+000D (CARRIAGE RETURN),
and U+000A (LINE FEED). Any code points may appear in the form of an escape sequence. String literals
evaluate to ECMAScript String values. When generating these String values Unicode code points are UTF-16
encoded as defined in 11.1.1. Code points belonging to the Basic
Multilingual Plane are encoded as a single code unit element of the string. All other code points are
encoded as two code unit elements of the string.
<LF> and <CR> cannot appear in a string literal, except as part of a LineContinuation to produce the empty
code points sequence. The proper way to include either in the String value of a string literal is to use
an escape sequence such as \n or \u000A.
It is possible for string literals to precede a Use Strict Directive that places the
enclosing code in strict mode, and implementations must take care to
enforce the above rules for such literals. For example, the following source text contains a Syntax
Error:
A string literal stands for a value of the String type. SV
produces String values for string literals through recursive application on the various parts of the
string literal. As part of this process, some Unicode code points within the string literal are
interpreted as having a mathematical value, as described below or in 12.9.3.
A regular expression literal is an input element that is converted to a RegExp object (see 22.2) each time the literal is
evaluated. Two regular expression literals in a program evaluate to regular expression objects that
never compare as === to each other even if the two literals' contents are identical. A
RegExp object may also be created at runtime by new RegExp or calling the RegExp constructor as a function
(see 22.2.4).
The productions below describe the syntax for a regular expression literal and are used by the input
element scanner to find the end of the regular expression literal. The source text comprising the RegularExpressionBody and the RegularExpressionFlags are
subsequently parsed again using the more stringent ECMAScript Regular Expression grammar (22.2.1).
An implementation may extend the ECMAScript Regular Expression grammar defined in 22.2.1, but it must not extend
the RegularExpressionBody and
RegularExpressionFlags
productions defined below or the productions used by these productions.
Regular expression literals may not be empty; instead of representing an empty regular expression
literal, the code unit sequence // starts a single-line comment. To specify an empty
regular expression, use: /(?:)/.
12.9.5.1 Static Semantics: BodyText
The syntax-directed operation
BodyText takes no arguments and returns source text. It is defined piecewise over the following
productions:
The syntax-directed operation
TV takes no arguments and returns a String or undefined. A template literal component
is interpreted by TV as a value of the String type. TV is
used to construct the indexed components of a template object (colloquially, the template values). In TV,
escape sequences are replaced by the UTF-16 code unit(s) of the Unicode code point represented by the
escape sequence.
The syntax-directed operation
TRV takes no arguments and returns a String. A template literal component is interpreted by TRV as a value
of the String type. TRV is used to construct
the raw components of a template object (colloquially, the template raw values). TRV is similar to
TV with the
difference being that in TRV, escape sequences are interpreted as they appear in the literal.
The TRV of HexDigit::one
of0123456789abcdefABCDEF is the result of performing UTF16EncodeCodePoint on the single code point
matched by this production.
Most ECMAScript statements and declarations must be terminated with a semicolon. Such semicolons may always
appear explicitly in the source text. For convenience, however, such semicolons may be omitted from the source
text in certain situations. These situations are described by saying that semicolons are automatically
inserted into the source code token stream in those situations.
12.10.1 Rules of Automatic Semicolon Insertion
In the following rules, “token” means the actual recognized lexical token determined using the current
lexical goal symbol as described in clause 12.
There are three basic rules of semicolon insertion:
When, as the source text is parsed from left to right, a token (called the offending token) is
encountered that is not allowed by any production of the grammar, then a semicolon is automatically
inserted before the offending token if one or more of the following conditions is true:
The offending token is separated from the previous token by at least one LineTerminator.
The offending token is }.
The previous token is ) and the inserted semicolon would then be parsed as the
terminating semicolon of a do-while statement (14.7.2).
When, as the source text is parsed from left to right, the end of the input stream of tokens is
encountered and the parser is unable to parse the input token stream as a single instance of the goal
nonterminal, then a semicolon is automatically inserted at the end of the input stream.
When, as the source text is parsed from left to right, a token is encountered that is allowed by some
production of the grammar, but the production is a restricted production and the token would be
the first token for a terminal or nonterminal immediately following the annotation “[no LineTerminator here]” within the restricted
production (and therefore such a token is called a restricted token), and the restricted token is
separated from the previous token by at least one LineTerminator, then a semicolon is automatically inserted
before the restricted token.
However, there is an additional overriding condition on the preceding rules: a semicolon is never inserted
automatically if the semicolon would then be parsed as an empty statement or if that semicolon would become
one of the two semicolons in the header of a for statement (see 14.7.4).
Note
The following are the only restricted productions in the grammar:
The practical effect of these restricted productions is as follows:
When a ++ or -- token is encountered where the parser would treat it as a
postfix operator, and at least one LineTerminator occurred between the preceding token and
the ++ or -- token, then a semicolon is automatically inserted before the
++ or -- token.
When a continue, break, return, throw, or
yield token is encountered and a LineTerminator is encountered before the next token, a
semicolon is automatically inserted after the continue, break,
return, throw, or yield token.
When arrow function parameter(s) are followed by a LineTerminator before a => token, a
semicolon is automatically inserted and the punctuator causes a syntax error.
When an async token is followed by a LineTerminator before a function or IdentifierName or ( token,
a semicolon is automatically inserted and the async token is not treated as part of the
same expression or class element as the following tokens.
When an async token is followed by a LineTerminator before a * token, a semicolon
is automatically inserted and the punctuator causes a syntax error.
The resulting practical advice to ECMAScript programmers is:
A postfix ++ or -- operator should be on the same line as its operand.
An Expression in a return
or throw statement or an AssignmentExpression in a yield
expression should start on the same line as the return, throw, or
yield token.
A LabelIdentifier in a
break or continue statement should be on the same line as the
break or continue token.
The end of an arrow function's parameter(s) and its => should be on the same line.
The async token preceding an asynchronous function or method should be on the same line
as the immediately following token.
12.10.2 Examples of Automatic Semicolon Insertion
This section is non-normative.
The source
{ 12 } 3
is not a valid sentence in the ECMAScript grammar, even with the automatic semicolon insertion rules. In
contrast, the source
{ 12 } 3
is also not a valid ECMAScript sentence, but is transformed by automatic semicolon insertion into the
following:
{ 1
;2 ;} 3;
which is a valid ECMAScript sentence.
The source
for (a; b
)
is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion because the
semicolon is needed for the header of a for statement. Automatic semicolon insertion never
inserts one of the two semicolons in the header of a for statement.
The source
return
a + b
is transformed by automatic semicolon insertion into the following:
return;
a + b;
Note 1
The expression a + b is not treated as a value to be returned by the return
statement, because a LineTerminator
separates it from the token return.
The source
a = b
++c
is transformed by automatic semicolon insertion into the following:
a = b;
++c;
Note 2
The token ++ is not treated as a postfix operator applying to the variable b,
because a LineTerminator occurs
between b and ++.
The source
if (a > b)
else c = d
is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion before the
else token, even though no production of the grammar applies at that point, because an
automatically inserted semicolon would then be parsed as an empty statement.
The source
a = b + c
(d + e).print()
is not transformed by automatic semicolon insertion, because the parenthesized expression that
begins the second line can be interpreted as an argument list for a function call:
a = b + c(d + e).print()
In the circumstance that an assignment statement must begin with a left parenthesis, it is a good idea for
the programmer to provide an explicit semicolon at the end of the preceding statement rather than to rely on
automatic semicolon insertion.
12.10.3 Interesting Cases of Automatic Semicolon Insertion
This section is non-normative.
ECMAScript programs can be written in a style with very few semicolons by relying on automatic semicolon
insertion. As described above, semicolons are not inserted at every newline, and automatic semicolon
insertion can depend on multiple tokens across line terminators.
As new syntactic features are added to ECMAScript, additional grammar productions could be added that cause
lines relying on automatic semicolon insertion preceding them to change grammar productions when parsed.
For the purposes of this section, a case of automatic semicolon insertion is considered interesting if it
is a place where a semicolon may or may not be inserted, depending on the source text which precedes it. The
rest of this section describes a number of interesting cases of automatic semicolon insertion in this
version of ECMAScript.
12.10.3.1 Interesting Cases of Automatic Semicolon Insertion in Statement
Lists
In a StatementList, many StatementListItems end in semicolons,
which may be omitted using automatic semicolon insertion. As a consequence of the rules above, at the end
of a line ending an expression, a semicolon is required if the following line begins with any of the
following:
An opening parenthesis ((). Without a semicolon, the two lines together
are treated as a CallExpression.
An opening square bracket ([). Without a semicolon, the two lines
together are treated as property access, rather than an ArrayLiteral or ArrayAssignmentPattern.
A template literal (`). Without a semicolon, the two lines together are
interpreted as a tagged Template (13.3.11), with the previous expression as the MemberExpression.
Unary + or -. Without a semicolon, the two lines together
are interpreted as a usage of the corresponding binary operator.
A RegExp literal. Without a semicolon, the two lines together may be parsed instead
as the /MultiplicativeOperator, for example if the RegExp
has flags.
12.10.3.2 Cases of Automatic Semicolon Insertion and “[no LineTerminator here]”
This section is non-normative.
ECMAScript contains grammar productions which include “[no LineTerminator here]”. These productions are sometimes a
means to have optional operands in the grammar. Introducing a LineTerminator in these locations would change the grammar
production of a source text by using the grammar production without the optional operand.
The rest of this section describes a number of productions using “[no LineTerminator here]” in this version of ECMAScript.
12.10.3.2.1 List of Grammar Productions with Optional Operands and “[no
LineTerminator here]”
yield and await are permitted as BindingIdentifier in the grammar, and prohibited with
static
semantics below, to prohibit automatic semicolon insertion in cases such as
It is a Syntax Error if IsStrict(this phrase) is true and the StringValue
of IdentifierName is one of
"implements", "interface", "let",
"package", "private", "protected",
"public", "static", or "yield".
An ArrayLiteral is an expression
describing the initialization of an Array, using a list, of zero or more expressions each of which
represents an array element, enclosed in square brackets. The elements need not be literals; they are
evaluated each time the array initializer is evaluated.
Array elements may be elided at the beginning, middle or end of the element list. Whenever a comma in the
element list is not preceded by an AssignmentExpression (i.e., a comma at the beginning or
after another comma), the missing array element contributes to the length of the Array and increases the
index of subsequent elements. Elided array elements are not defined. If an element is elided at the end of
an array, that element does not contribute to the length of the Array.
CreateDataPropertyOrThrow is used to ensure
that own properties are defined for the array even if the standard built-in Array prototype object has
been modified in a manner that would preclude the creation of new own properties using [[Set]].
An object initializer is an expression describing the initialization of an Object, written in a form
resembling a literal. It is a list of zero or more pairs of property keys and associated values, enclosed in
curly brackets. The values need not be literals; they are evaluated each time the object initializer is
evaluated.
In certain contexts, ObjectLiteral
is used as a cover grammar for a more restricted secondary grammar. The CoverInitializedName production is necessary to fully
cover these secondary grammars. However, use of this production results in an early Syntax Error in
normal contexts where an actual ObjectLiteral is expected.
It is a Syntax Error if any source text is matched by this production.
Note 1
This production exists so that ObjectLiteral can serve as a cover grammar for ObjectAssignmentPattern. It
cannot occur in an actual object initializer.
The syntax-directed operation
PropertyNameList takes no arguments and returns a List of Strings. It is
defined piecewise over the following productions:
The abstract operation IsValidRegularExpressionLiteral takes argument literal (a RegularExpressionLiteralParse Node)
and returns a Boolean. It determines if its argument is a valid regular expression literal. It performs
the following steps when called:
Set patternText to the sequence of code points resulting from interpreting each of
the 16-bit elements of stringValue as a Unicode BMP code point. UTF-16 decoding is not
applied to the elements.
Let parseResult be ParsePattern(patternText, u,
v).
If parseResult is a Parse Node, return true; else
return false.
The syntax-directed operation
TemplateStrings takes argument raw (a Boolean) and returns a List of either Strings or
undefined. It is defined piecewise over the following productions:
This operation returns undefined if raw is false and
templateToken contains a NotEscapeSequence. In all other cases, it returns a
String.
13.2.8.4 GetTemplateObject ( templateLiteral )
The abstract operation GetTemplateObject takes argument templateLiteral (a Parse Node)
and returns an Array. It performs the following steps when called:
Append the Record { [[Site]]: templateLiteral, [[Array]]:
template } to realm.[[TemplateMap]].
Return template.
Note 1
The creation of a template object cannot result in an abrupt completion.
Note 2
Each TemplateLiteral in the
program code of a realm is
associated with a unique template object that is used in the evaluation of tagged Templates (13.2.8.6). The template
objects are frozen and the same template object is used each time a specific tagged Template is
evaluated. Whether template objects are created lazily upon first evaluation of the TemplateLiteral or eagerly prior to
first evaluation is an implementation choice that is not observable to ECMAScript code.
Note 3
Future editions of this specification may define additional non-enumerable properties of template
objects.
This algorithm does not apply GetValue to Evaluation of Expression. The principal motivation for this is so that
operators such as delete and typeof may be applied to parenthesized
expressions.
Let propertyNameReference be ? Evaluation of expression.
Let propertyNameValue be ? GetValue(propertyNameReference).
NOTE: In most cases, ToPropertyKey will be performed on
propertyNameValue immediately after this step. However, in the case of a[b] = c,
it will not be performed until after evaluation of c.
Return the Reference Record { [[Base]]: baseValue, [[ReferencedName]]:
propertyNameValue, [[Strict]]: strict, [[ThisValue]]: empty }.
The abstract operation EvaluatePropertyAccessWithIdentifierKey takes arguments baseValue (an
ECMAScript language value),
identifierName (an IdentifierNameParse Node), and strict (a
Boolean) and returns a Reference Record. It performs the
following steps when called:
Let propertyNameString be the StringValue of identifierName.
Return the Reference Record { [[Base]]: baseValue, [[ReferencedName]]:
propertyNameString, [[Strict]]: strict, [[ThisValue]]: empty }.
NOTE: In most cases, ToPropertyKey will be performed on
propertyNameValue immediately after this step. However, in the case of
super[b] = c, it will not be performed until after evaluation of c.
The abstract operation EvaluateImportCall takes argument specifierExpression (a Parse Node)
and optional argument optionsExpression (a Parse Node) and returns either a
normal completion containing a Promise
or an abrupt completion. It performs the
following steps when called:
Perform ! Call(promiseCapability.[[Reject]], undefined, « a newly created
TypeError object »).
Return promiseCapability.[[Promise]].
Sort attributes according to the lexicographic order of their [[Key]] field, treating the value of each such field as a sequence of UTF-16
code unit values. NOTE: This sorting is observable only in that hosts are prohibited from changing behaviour based
on the order in which attributes are enumerated.
Let moduleRequest be a new ModuleRequest Record { [[Specifier]]: specifierString, [[Attributes]]: attributes }.
The abstract operation ContinueDynamicImport takes arguments promiseCapability (a PromiseCapability Record) and
moduleCompletion (either a normal completion
containing a Module Record or a throw completion) and returns
unused. It completes the process of a dynamic import originally started by an
import() call, resolving or rejecting the promise
returned by that call as appropriate. It performs the following steps when called:
Let linkAndEvaluateClosure be a new Abstract Closure with no parameters
that captures module, promiseCapability, and onRejected and performs
the following steps when called:
Let fulfilledClosure be a new Abstract Closure with no
parameters that captures module and promiseCapability and performs the
following steps when called:
A tagged template is a function call where the arguments of the call are derived from a TemplateLiteral (13.2.8).
The actual arguments include a template object (13.2.8.4) and the values produced by evaluating the
expressions embedded within the TemplateLiteral.
The host-defined
abstract operation HostFinalizeImportMeta takes arguments importMeta (an Object) and
moduleRecord (a Module Record) and returns
unused. It allows hosts to perform any extraordinary operations to prepare the object
returned from import.meta.
Most hosts will be able to simply
define HostGetImportMetaProperties, and leave
HostFinalizeImportMeta with its default behaviour. However, HostFinalizeImportMeta provides an "escape
hatch" for hosts which need to
directly manipulate the object before it is exposed to ECMAScript code.
The default implementation of HostFinalizeImportMeta is to return unused.
When a delete operator occurs within strict mode code, a
SyntaxError exception is thrown if its UnaryExpression is a direct reference to a variable,
function argument, or function name. In addition, if a delete operator occurs within
strict mode
code and the property to be deleted has the attribute { [[Configurable]]: false } (or otherwise cannot be deleted), a
TypeError exception is thrown.
Note 2
The object that may be created in step 4.c is not accessible outside
of the above abstract operation and the ordinary object[[Delete]]
internal method. An implementation might choose to avoid the actual creation of that object.
The result of evaluating a relational operator is always of type Boolean, reflecting whether the
relationship named by the operator holds between its two operands.
The abstract operation InstanceofOperator takes arguments V (an ECMAScript
language value) and target (an ECMAScript language value) and
returns either a normal completion containing a Boolean
or a throw completion. It implements the
generic algorithm for determining if V is an instance of target either by consulting
target's %Symbol.hasInstance% method or, if absent, determining
whether the value of target's "prototype" property is present in
V's prototype chain. It performs the following steps when called:
Steps 4 and 5
provide compatibility with previous editions of ECMAScript that did not use a %Symbol.hasInstance% method to define the
instanceof operator semantics. If an object does not define or inherit %Symbol.hasInstance% it uses the default
instanceof semantics.
13.11 Equality Operators
Note
The result of evaluating an equality operator is always of type Boolean, reflecting whether the
relationship named by the operator holds between its two operands.
If r is true, return false; otherwise return
true.
Note 1
Given the above definition of equality:
String comparison can be forced by: `${a}` == `${b}`.
Numeric comparison can be forced by: +a == +b.
Boolean comparison can be forced by: !a == !b.
Note 2
The equality operators maintain the following invariants:
A != B is equivalent to !(A == B).
A == B is equivalent to B == A, except in the order of evaluation of
A and B.
Note 3
The equality operator is not always transitive. For example, there might be two distinct String
objects, each representing the same String value; each String object would be considered equal to the
String value by the == operator, but the two String objects would not be equal to each
other. For example:
new String("a") == "a" and "a" == new String("a") are both
true.
new String("a") == new String("a") is false.
Note 4
Comparison of Strings uses a simple equality test on sequences of code unit values. There is no attempt
to use the more complex, semantically oriented definitions of character or string equality and collating
order defined in the Unicode specification. Therefore Strings values that are canonically equal
according to the Unicode Standard could test as unequal. In effect this algorithm assumes that both
Strings are already in normalized form.
The value produced by a && or || operator is not necessarily of type
Boolean. The value produced will always be the value of one of the two operand expressions.
The grammar for a ConditionalExpression in ECMAScript is slightly
different from that in C and Java, which each allow the second subexpression to be an Expression but restrict the third expression to
be a ConditionalExpression. The
motivation for this difference in ECMAScript is to allow an assignment expression to be governed by either
arm of a conditional and to eliminate the confusing and fairly useless case of a comma expression as the
centre expression.
When this expression occurs within strict mode code, it is a runtime error if
lRef in step 1.e,
3, 2, 2, 2 is an
unresolvable reference. If it is, a ReferenceError exception is thrown. Additionally,
it is a runtime error if the lRef in step 9, 6, 6, 6 is a reference
to a data
property with the attribute value { [[Writable]]:
false }, to an accessor property with the attribute value { [[Set]]: undefined }, or to a non-existent property of an
object for which the IsExtensible predicate returns the value
false. In these cases a TypeError exception is thrown.
No hint is provided in the calls to ToPrimitive in steps 1.a and 1.b. All standard objects except Dates handle
the absence of a hint as if number were given; Dates handle the absence of a hint
as if string were given. Exotic objects may handle the absence of a hint in some other
manner.
Note 2
Step 1.c differs from step 3 of the
IsLessThan
algorithm, by using the logical-or operation instead of the logical-and operation.
The abstract operation EvaluateStringOrNumericBinaryExpression takes arguments leftOperand (a
Parse
Node), opText (a sequence of Unicode code points), and rightOperand
(a Parse
Node) and returns either a normal completion
containing either a String, a BigInt, or a Number, or an abrupt completion. It performs the
following steps when called:
The value of a StatementList is the
value of the last value-producing item in the StatementList. For example, the following calls to the
eval function all return the value 1:
The abstract operation BlockDeclarationInstantiation takes arguments code (a Parse Node)
and env (a Declarative Environment Record) and returns
unused. code is the Parse Node corresponding to the body of the
block. env is the Environment Record in which bindings are to be created.
When undefined is passed for environment it indicates that a PutValue operation should be
used to assign the initialization value. This is the case for formal parameter lists of non-strict
functions. In that case the formal parameter bindings are preinitialized in order
to deal with the possibility of multiple parameters with the same name.
It is defined piecewise over the following productions:
The lookahead-restriction [lookahead ≠ else] resolves the classic
"dangling else" problem in the usual way. That is, when the choice of associated if is
otherwise ambiguous, the else is associated with the nearest (innermost) of the candidate
ifs
The abstract operation LoopContinues takes arguments completion (a Completion Record) and
labelSet (a List of Strings) and returns a Boolean.
It performs the following steps when called:
The abstract operation CreatePerIterationEnvironment takes argument perIterationBindings (a
List of Strings) and returns either a
normal completion containingunused or a throw completion. It
performs the following steps when called:
undefined is passed for environment to indicate that a PutValue operation should be
used to assign the initialization value. This is the case for var statements and the
formal parameter lists of some non-strict functions (see 10.2.11). In those cases a lexical
binding is hoisted and preinitialized prior to evaluation of its initializer.
It is defined piecewise over the following productions:
The abstract operation ForIn/OfBodyEvaluation takes arguments lhs (a Parse
Node), stmt (a StatementParse Node), iteratorRecord
(an Iterator
Record), iterationKind (enumerate or
iterate), lhsKind (assignment,
var-binding, or lexical-binding), and labelSet (a
List of Strings) and optional argument
iteratorKind (sync or async) and returns either a
normal completion containing an
ECMAScript language value or an abrupt completion. It performs the
following steps when called:
If iteratorKind is not present, set iteratorKind to
sync.
The abstract operation EnumerateObjectProperties takes argument O (an Object) and returns an
iterator
object. It performs the following steps when called:
Return an iterator object whose next method
iterates over all the String-valued keys of enumerable properties of O. The iterator
object is never directly accessible to ECMAScript code. The mechanics and order of
enumerating the properties is not specified but must conform to the rules specified below.
The iterator's throw and return
methods are null and are never invoked. The iterator's next method
processes object properties to determine whether the property key should be returned as an iterator
value. Returned property
keys do not include keys that are Symbols. Properties of the target object may be
deleted during enumeration. A property that is deleted before it is processed by the iterator's next method is ignored. If new
properties are added to the target object during enumeration, the newly added properties are not
guaranteed to be processed in the active enumeration. A property name will be returned by the iterator's next method at most once in any
enumeration.
Enumerating the properties of the target object includes enumerating properties of its prototype, and the
prototype of the prototype, and so on, recursively; but a property of a prototype is not processed if it
has the same name as a property that has already been processed by the iterator's next method. The values of [[Enumerable]] attributes are not considered when determining if a property of a
prototype object has already been processed. The enumerable property names of prototype objects must be
obtained by invoking EnumerateObjectProperties passing the prototype object as the argument.
EnumerateObjectProperties must obtain the own property keys of the target object by calling its [[OwnPropertyKeys]] internal method. Property attributes of the target object must
be obtained by calling its [[GetOwnProperty]] internal method.
the value of the [[Prototype]] internal slot of O or an object in
its prototype chain changes,
a property is removed from O or an object in its prototype chain,
a property is added to an object in O's prototype chain, or
the value of the [[Enumerable]] attribute of a property of O or an
object in its prototype chain changes.
Note 1
ECMAScript implementations are not required to implement the algorithm in 14.7.5.10.2.1 directly. They may choose
any implementation whose behaviour will not deviate from that algorithm unless one of the constraints
in the previous paragraph is violated.
The following is an informative definition of an ECMAScript generator function that conforms to these
rules:
function* EnumerateObjectProperties(obj) {
const visited = newSet();
for (const key ofReflect.ownKeys(obj)) {
if (typeof key === "symbol") continue;
const desc = Reflect.getOwnPropertyDescriptor(obj, key);
if (desc) {
visited.add(key);
if (desc.enumerable) yield key;
}
}
const proto = Reflect.getPrototypeOf(obj);
if (proto === null) return;
for (const protoKey ofEnumerateObjectProperties(proto)) {
if (!visited.has(protoKey)) yield protoKey;
}
}
Note 2
The list of exotic
objects for which implementations are not required to match CreateForInIterator was chosen because
implementations historically differed in behaviour for those cases, and agreed in all others.
14.7.5.10 For-In Iterator Objects
A For-In Iterator is
an object that represents a specific iteration over some specific object. For-In Iterator objects are
never directly accessible to ECMAScript code; they exist solely to illustrate the behaviour of EnumerateObjectProperties.
14.7.5.10.1 CreateForInIterator ( object )
The abstract operation CreateForInIterator takes argument object (an Object) and returns a
For-In Iterator. It is used to create a For-In
Iterator object which iterates over the own and inherited enumerable string
properties of object in a specific order. It performs the following steps when called:
It is a Syntax Error if this ContinueStatement is not nested, directly or indirectly
(but not crossing function or static initialization block boundaries), within an IterationStatement.
It is a Syntax Error if this BreakStatement is not nested, directly or indirectly (but not
crossing function or static initialization block boundaries), within an IterationStatement or a SwitchStatement.
A return statement causes a function to cease execution and, in most cases, returns a value
to the caller. If Expression is omitted,
the return value is undefined. Otherwise, the return value is the value of Expression. A return statement may
not actually return a value to the caller depending on surrounding context. For example, in a
try block, a return statement's Completion Record may be replaced with
another Completion Record during evaluation of
the finally block.
The with statement adds an Object Environment Record for a
computed object to the lexical environment of the running execution context. It then
executes a statement using this augmented lexical environment. Finally, it restores the original lexical
environment.
No matter how control leaves the embedded Statement, whether normally or by some form of abrupt completion or exception, the
LexicalEnvironment is always restored to its former state.
This operation does not execute C's StatementList (if any). The CaseBlock algorithm uses its return value to determine which
StatementList to start executing.
A Statement may be prefixed by a label.
Labelled statements are only used in conjunction with labelled break and
continue statements. ECMAScript has no goto statement. A Statement can be part of a LabelledStatement, which itself can be
part of a LabelledStatement, and so
on. The labels introduced this way are collectively referred to as the “current label set” when describing
the semantics of individual statements.
The abstract operation IsLabelledFunction takes argument stmt (a StatementParse Node) and returns a Boolean. It performs the
following steps when called:
The try statement encloses a block of code in which an exceptional condition can occur, such
as a runtime error or a throw statement. The catch clause provides the
exception-handling code. When a catch clause catches an exception, its CatchParameter is bound to that exception.
Evaluating a DebuggerStatement
may allow an implementation to cause a breakpoint when run under a debugger. If a debugger is not
present or active this statement has no observable effect.
Various ECMAScript language elements cause the creation of ECMAScript function objects (10.2). Evaluation of such functions starts with the
execution of their [[Call]] internal method (10.2.1).
The syntax-directed operation
ExpectedArgumentCount takes no arguments and returns a non-negative integer. It is defined piecewise over the following
productions:
The ExpectedArgumentCount of a FormalParameterList is the number of FormalParameters to the left of either
the rest parameter or the first FormalParameter with an Initializer. A FormalParameter without an initializer is
allowed after the first parameter with an initializer but such parameters are considered to be optional
with undefined as their default value.
The syntax-directed operation
FunctionBodyContainsUseStrict takes no arguments and returns a Boolean. It is defined piecewise over the
following productions:
A "prototype" property is automatically created for every function defined using a
FunctionDeclaration or FunctionExpression, to allow for the
possibility that the function will be used as a constructor.
The syntax-directed operation
ConciseBodyContainsUseStrict takes no arguments and returns a Boolean. It is defined piecewise over the
following productions:
An ArrowFunction does not define
local bindings for arguments, super, this, or
new.target. Any reference to arguments, super, this,
or new.target within an ArrowFunction must resolve to a binding in a lexically
enclosing environment. Typically this will be the Function Environment of an immediately enclosing
function. Even though an ArrowFunction may contain references to super,
the function
object created in step 5 is not made
into a method by performing MakeMethod. An ArrowFunction that references super is always
contained within a non-ArrowFunction
and the necessary state to implement super is accessible via the env that is
captured by the function
object of the ArrowFunction.
YieldExpression cannot be used
within the FormalParameters of a
generator function because any expressions that are part of FormalParameters are evaluated before the resulting
Generator is in a resumable state.
Let innerResult be ? Call(throw,
iterator, « received.[[Value]] »).
If generatorKind is async, set innerResult to
? Await(innerResult).
NOTE: Exceptions from the inner iteratorthrow method are
propagated. Normal completions from
an inner throw method are processed similarly to an inner next.
NOTE: If iterator does not have a throw method, this throw is
going to terminate the yield* loop. But first we need to give
iterator a chance to clean up.
It is a Syntax Error if the PrivateBoundIdentifiers of ClassElementList contains any duplicate
entries, unless the name is used once for a getter and once for a setter and in no other entries, and the
getter and setter are either both static or both non-static.
The syntax-directed operation
ClassElementKind takes no arguments and returns constructor-method,
non-constructor-method, or empty. It is defined piecewise over
the following productions:
The syntax-directed operation
AllPrivateIdentifiersValid takes argument names (a List of Strings) and returns a Boolean.
Every grammar production alternative in this specification which is not listed below implicitly has the
following default definition of AllPrivateIdentifiersValid:
The syntax-directed operation
PrivateBoundIdentifiers takes no arguments and returns a List of Strings. It is defined piecewise
over the following productions:
Every grammar production alternative in this specification which is not listed below implicitly has the
following default definition of ContainsArguments:
For ease of specification, private methods and accessors are included alongside private fields in the
[[PrivateElements]] slot of class instances. However, any given object has
either all or none of the private methods and accessors defined by a given class. This feature has been
designed so that implementations may choose to implement private methods and accessors using a strategy
which does not require tracking each method or accessor individually.
For example, an implementation could directly associate instance private methods with their
corresponding Private
Name and track, for each object, which class constructors have run with that object as their
this value. Looking up an instance private method on an object then consists of checking
that the class constructor which defines the method has been used to initialize
the object, then returning the method associated with the Private Name.
This differs from private fields: because field initializers can throw during class instantiation, an
individual object may have some proper subset of the private fields of a given class, and so private
fields must in general be tracked individually.
It is defined piecewise over the following productions:
NOTE: This branch behaves similarly to
constructor(...args) { super(...args); }. The most notable distinction is
that while the aforementioned ECMAScript source text observably calls the
%Symbol.iterator% method on
%Array.prototype%, this function does not.
Let func be ! F.[[GetPrototypeOf]]().
If IsConstructor(func) is
false, throw a TypeError exception.
await is parsed as a keyword of an AwaitExpression when the [Await] parameter is
present. The [Await] parameter is present in the top level of the following contexts, although
the parameter may be absent in some contexts depending on the nonterminals, such as FunctionBody:
When Script is the syntactic goal
symbol, await may be parsed as an identifier when the [Await]
parameter is absent. This includes the following contexts:
The syntax-directed operation
AsyncConciseBodyContainsUseStrict takes no arguments and returns a Boolean. It is defined piecewise over the
following productions:
Tail Position calls are only defined in strict mode code because of a common non-standard
language extension (see 10.2.4) that enables observation of the
chain of caller contexts.
call is a Parse Node that represents a specific range of source
text. When the following algorithms compare call to another Parse
Node, it is a test of whether they represent the same source text.
Note 2
A potential tail position call that is immediately followed by return GetValue of the call result is also a possible
tail position call. A function call cannot return a Reference Record, so such a GetValue operation will always
return the same value as the actual function call result.
It is defined piecewise over the following productions:
The abstract operation PrepareForTailCall takes no arguments and returns unused. It
performs the following steps when called:
Assert: The current execution
context will not subsequently be used for the evaluation of any ECMAScript code or
built-in functions. The invocation of Call subsequent to the invocation of this abstract operation will
create and push a new execution context before performing any such
evaluation.
Discard all resources associated with the current execution context.
Return unused.
A tail position call must either release any transient internal resources associated with the currently
executing function execution context before invoking the target function or
reuse those resources in support of the target function.
Note
For example, a tail position call should only grow an implementation's activation record stack by the
amount that the size of the target function's activation record exceeds the size of the calling
function's activation record. If the target function's activation record is smaller, then the total size
of the stack should decrease.
A map from the specifier strings imported by this script to the resolved Module
Record. The list does not contain two different Recordsr1 and
r2 such that ModuleRequestsEqual(r1,
r2) is true.
[[HostDefined]]
anything (default value is empty)
Field reserved for use by host environments that need to associate additional
information with a script.
The abstract operation ParseScript takes arguments sourceText (ECMAScript source text), realm (a
Realm Record), and
hostDefined (anything) and returns a Script Record or a non-empty List of SyntaxError
objects. It creates a Script
Record based upon the result of parsing sourceText as a Script. It performs the following steps when called:
An implementation may parse script source text and analyse it for Early Error conditions prior to
evaluation of ParseScript for that script source text. However, the reporting of any errors must be
deferred until the point where this specification actually performs ParseScript upon that source text.
When an execution context is established for evaluating
scripts, declarations are instantiated in the current global environment. Each global binding declared
in the code is instantiated.
NOTE: Global var and function bindings (except those that are introduced
by non-strict direct eval) are
non-configurable and are therefore restricted global properties.
If hasRestrictedGlobal is true, throw a
SyntaxError exception.
If vnDefinable is false, throw a
TypeError exception.
If declaredVarNames does not contain vn, then
Append vn to declaredVarNames.
NOTE: No abnormal terminations occur after this algorithm step if the global object is
an ordinary
object. However, if the global object is a Proxy exotic object it may exhibit
behaviours that cause abnormal terminations in some of the following steps.
NOTE: Annex B.3.2.2 adds additional steps
at this point.
Early errors
specified in 16.1.1 prevent name conflicts
between function/var declarations and let/const/class declarations as well as redeclaration of
let/const/class bindings for declaration contained within a single Script. However, such conflicts and redeclarations that span more
than one Script are detected as runtime
errors during GlobalDeclarationInstantiation. If any such errors are detected, no bindings are
instantiated for the script. However, if the global object is defined using Proxy exotic
objects then the runtime tests for conflicting declarations may be unreliable
resulting in an abrupt completion and some global
declarations not being instantiated. If this occurs, the code for the Script is not evaluated.
Unlike explicit var or function declarations, properties that are directly created on the global object
result in global bindings that may be shadowed by let/const/class declarations.
The duplicate ExportedNames rule implies that multiple
export defaultExportDeclaration items within a ModuleBody is a Syntax Error. Additional
error conditions relating to conflicting or duplicate declarations are checked during module linking
prior to evaluation of a Module. If any
such errors are detected the Module is not
evaluated.
The abstract operation ImportedLocalNames takes argument importEntries (a List of ImportEntry
Records) and returns a List of Strings. It
creates a List of all of the local name bindings
defined by importEntries. It performs the following steps when called:
A LoadedModuleRequest Record represents the request to import a module together with
the resulting Module Record. It consists of the same fields
defined in table Table 38, with the addition of [[Module]]:
A Module Record encapsulates structural information
about the imports and exports of a single module. This information is used to link the imports and exports
of sets of connected modules. A Module Record includes four fields that are only used when evaluating a
module.
For specification purposes Module Record values are values of the Record specification type and can be
thought of as existing in a simple object-oriented hierarchy where Module Record is an abstract class with
both abstract and concrete subclasses. This specification defines the abstract subclass named Cyclic Module
Record and its concrete subclass named Source Text Module Record. Other
specifications and implementations may define additional Module Record subclasses corresponding to
alternative module definition facilities that they defined.
Module Record defines the fields listed in Table 41. All Module Definition subclasses include
at least those fields. Module Record also defines the abstract method list in Table 42. All Module definition
subclasses must provide concrete implementations of these abstract methods.
Prepares the module for linking by recursively loading all its dependencies, and returns a
promise.
GetExportedNames([exportStarSet])
Return a list of all names that are either directly or indirectly exported from this module.
LoadRequestedModules must have completed successfully prior to invoking this method.
ResolveExport(exportName [, resolveSet])
Return the binding of a name exported by this module. Bindings are represented by a ResolvedBinding
Record, of the form { [[Module]]: Module Record, [[BindingName]]: String | namespace }. If the
export is a Module Namespace Object without a direct binding in any module, [[BindingName]] will be set to namespace. Return
null if the name cannot be resolved, or ambiguous if
multiple bindings were found.
Each time this operation is called with a specific exportName,
resolveSet pair as arguments it must return the same result.
LoadRequestedModules must have completed successfully prior to invoking this method.
Link()
Prepare the module for evaluation by transitively resolving all module dependencies and
creating a Module Environment Record.
LoadRequestedModules must have completed successfully prior to invoking this method.
Evaluate()
Returns a promise for the evaluation of this module and its dependencies, resolving on
successful evaluation or if it has already been evaluated successfully, and rejecting for an
evaluation error or if it has already been evaluated unsuccessfully. If the promise is
rejected, hosts are
expected to handle the promise rejection and rethrow the evaluation error.
Link must have completed successfully prior to invoking this method.
16.2.1.5.1 EvaluateModuleSync ( module )
The abstract operation EvaluateModuleSync takes argument module (a Module
Record) and returns either a normal completion containingunused or a throw completion. It
synchronously evaluates module, provided that the caller guarantees that module's
evaluation will return an already settled promise. It performs the following steps when called:
A Cyclic Module
Record is used to represent information about a module that can participate in dependency cycles
with other modules that are subclasses of the Cyclic Module Record type. Module
Records that are not subclasses of the Cyclic Module Record type must not
participate in dependency cycles with Source Text Module Records.
new, unlinked, linking,
linked, evaluating,
evaluating-async, or evaluated
Initially new. Transitions to unlinked,
linking, linked,
evaluating, possibly evaluating-async,
evaluated (in that order) as the module progresses throughout its
lifecycle. evaluating-async indicates this module is queued to execute on
completion of its asynchronous dependencies or it is a module whose [[HasTLA]] field is true that has been executed and is
pending top-level completion.
Auxiliary field used during Link and Evaluate only. If [[Status]] is
either linking or evaluating, this is either the
module's depth-first traversal index or that of an "earlier" module in the same strongly
connected component.
A map from the specifier strings used by the module represented by this record to request the
importation of a module with the relative import attributes to the resolved Module Record. The list does not contain
two different Recordsr1 and
r2 such that ModuleRequestsEqual(r1,
r2) is true.
The first visited module of the cycle, the root DFS ancestor of the strongly connected
component. For a module not in a cycle, this would be the module itself. Once Evaluate has
completed, a module's [[DFSAncestorIndex]] is the depth-first traversal
index of its [[CycleRoot]].
[[HasTLA]]
a Boolean
Whether this module is individually asynchronous (for example, if it's a Source Text
Module Record containing a top-level await). Having an asynchronous
dependency does not mean this field is true. This field must not change after
the module is parsed.
This field is initially set to unset, and remains
unset for fully synchronous modules. For modules that are either
themselves asynchronous or have an asynchronous dependency, it is set to an integer that determines the
order in which execution of pending modules is queued by 16.2.1.6.1.3.4. Once the pending
module is executed, the field is set to done.
If this module is the [[CycleRoot]] of some cycle, and Evaluate() was
called on some module in that cycle, this field contains the PromiseCapability Record for that
entire evaluation. It is used to settle the Promise object that is returned from the Evaluate()
abstract method. This field will be empty for any dependencies of that
module, unless a top-level Evaluate() has been initiated for some of those dependencies.
If this module or a dependency has [[HasTLA]]true,
and execution is in progress, this tracks the parent importers of this module for the top-level
execution job. These parent modules will not start executing before this module has successfully
completed execution.
If this module has any asynchronous dependencies, this tracks the number of asynchronous
dependency modules remaining to execute for this module. A module with asynchronous dependencies
will be executed when this field reaches 0 and there are no execution errors.
Evaluate the module's code within its execution context. If this module has
true in [[HasTLA]], then a PromiseCapability Record is passed as
an argument, and the method is expected to resolve or reject the given capability. In this case,
the method must not throw an exception, but instead reject the PromiseCapability Record if necessary.
A GraphLoadingState
Record is a Record that contains information about
the loading process of a module graph. It's used to continue loading after a call to HostLoadImportedModule. Each GraphLoadingState
Record has the fields defined in Table 45:
It is a list of the Cyclic Module Records that have been already
loaded by the current loading process, to avoid infinite loops with circular dependencies.
The LoadRequestedModules concrete method of a Cyclic Module Recordmodule
takes optional argument hostDefined (anything) and returns a Promise. It populates the [[LoadedModules]] of all the Module Records in the
dependency graph of module (most of the work is done by the auxiliary function InnerModuleLoading). It takes an optional
hostDefined parameter that is passed to the HostLoadImportedModule hook. It
performs the following steps when called:
If hostDefined is not present, let hostDefined be
empty.
Let state be the GraphLoadingState Record { [[IsLoading]]: true, [[PendingModulesCount]]: 1, [[Visited]]: « »,
[[PromiseCapability]]: pc, [[HostDefined]]: hostDefined }.
The hostDefined parameter can be used to pass additional information necessary to fetch
the imported modules. It is used, for example, by HTML to set the correct fetch destination for
<link rel="preload" as="..."> tags.
import() expressions never set the hostDefined parameter.
The abstract operation InnerModuleLoading takes arguments state (a GraphLoadingState Record) and module
(a Module Record) and returns
unused. It is used by LoadRequestedModules to recursively perform the actual
loading process for module's dependency graph. It performs the following steps when
called:
The Link concrete method of a Cyclic Module Recordmodule takes no
arguments and returns either a normal completion
containingunused or a throw completion. On success, Link
transitions this module's [[Status]] from unlinked to
linked. On failure, an exception is thrown and this module's [[Status]] remains unlinked. (Most of the work is done by
the auxiliary function InnerModuleLinking.) It performs the following steps
when called:
Assert:
module.[[Status]] is one of unlinked,
linked, evaluating-async, or
evaluated.
16.2.1.6.1.2.1 InnerModuleLinking ( module,
stack, index )
The abstract operation InnerModuleLinking takes arguments module (a Module
Record), stack (a List of Cyclic Module
Records), and index (a non-negative integer) and returns either a normal completion containing a
non-negative integer or a
throw completion. It is used by
Link to perform the actual linking process for module, as well as recursively on all
other modules in the dependency graph. The stack and index parameters, as well
as a module's [[DFSAncestorIndex]] field, keep track of the depth-first
search (DFS) traversal. In particular, [[DFSAncestorIndex]] is used to
discover strongly connected components (SCCs), such that all modules in an SCC transition to
linked together. It performs the following steps when called:
If requiredModule and module are the same Module Record, set
done to true.
Return index.
16.2.1.6.1.3 Evaluate ( )
The Evaluate concrete method of a Cyclic Module Recordmodule takes no
arguments and returns a Promise. Evaluate transitions this module's [[Status]] from linked to either
evaluating-async or evaluated. The first time it is
called on a module in a given strongly connected component, Evaluate creates and returns a Promise
which resolves when the module has finished evaluating. This Promise is stored in the [[TopLevelCapability]] field of the [[CycleRoot]] for
the component. Future invocations of Evaluate on any module in the component return the same Promise.
(Most of the work is done by the auxiliary function InnerModuleEvaluation.) It
performs the following steps when called:
Assert: This call to
Evaluate is not happening at the same time as another call to Evaluate within the surrounding
agent.
Assert:
module.[[Status]] is one of linked,
evaluating-async, or evaluated.
If module.[[Status]] is either
evaluating-async or evaluated, set module
to module.[[CycleRoot]].
If module.[[TopLevelCapability]] is not
empty, then
16.2.1.6.1.3.1 InnerModuleEvaluation ( module,
stack, index )
The abstract operation InnerModuleEvaluation takes arguments module (a Module
Record), stack (a List of Cyclic Module
Records), and index (a non-negative integer) and returns either a normal completion containing a
non-negative integer or a
throw completion. It is used by
Evaluate to perform the actual evaluation process for module, as well as recursively on
all other modules in the dependency graph. The stack and index parameters, as
well as module's [[DFSAncestorIndex]] field, are used the same
way as in InnerModuleLinking. It performs the following
steps when called:
Assert:
requiredModule.[[AsyncEvaluationOrder]] is either an
integer or
unset.
If requiredModule.[[AsyncEvaluationOrder]] is
unset, set requiredModule.[[Status]] to evaluated.
Otherwise, set requiredModule.[[Status]] to
evaluating-async.
If requiredModule and module are the same Module Record, set
done to true.
Set requiredModule.[[CycleRoot]] to
module.
Return index.
Note 1
A module is evaluating while it is being traversed by
InnerModuleEvaluation. A module is evaluated on execution completion or
evaluating-async during execution if its [[HasTLA]] field is true or if it has asynchronous
dependencies.
Note 2
Any modules depending on a module of an asynchronous cycle when that cycle is not
evaluating will instead depend on the execution of the root of the cycle
via [[CycleRoot]]. This ensures that the cycle state can be treated as
a single strongly connected component through its root module state.
16.2.1.6.1.3.2 ExecuteAsyncModule ( module )
The abstract operation ExecuteAsyncModule takes argument module (a Cyclic Module
Record) and returns unused. It performs the following
steps when called:
Assert:
module.[[Status]] is either
evaluating or evaluating-async.
The abstract operation GatherAvailableAncestors takes arguments module (a Cyclic Module
Record) and execList (a List of Cyclic Module
Records) and returns unused. It performs the following
steps when called:
When an asynchronous execution for a root module is fulfilled, this function
determines the list of modules which are able to synchronously execute together on this
completion, populating them in execList.
The abstract operation AsyncModuleExecutionFulfilled takes argument module (a Cyclic Module
Record) and returns unused. It performs the following
steps when called:
Assert: All elements
of execList have their [[AsyncEvaluationOrder]] field set to
an integer, [[PendingAsyncDependencies]] field set to 0, and [[EvaluationError]] field set to empty.
Let sortedExecList be a List whose
elements are the elements of execList, sorted by their [[AsyncEvaluationOrder]] field in ascending order.
The abstract operation AsyncModuleExecutionRejected takes arguments module (a Cyclic Module
Record) and error (an ECMAScript language value)
and returns unused. It performs the following steps when called:
NOTE: module.[[AsyncEvaluationOrder]] is set to
done for symmetry with AsyncModuleExecutionFulfilled.
In InnerModuleEvaluation, the value of a
module's [[AsyncEvaluationOrder]] internal slot is unused when its [[EvaluationError]] internal slot is not empty.
This non-normative section gives a series of examples of the linking and evaluation of a few common
module graphs, with a specific focus on how errors can occur.
First consider the following simple module graph:
Figure 2: A simple module graph
Let's first assume that there are no error conditions. When a host first calls A.LoadRequestedModules(), this will complete
successfully by assumption, and recursively load the dependencies of B and C as
well (respectively, C and none), and then set A.[[Status]] = B.[[Status]] = C.[[Status]] = unlinked. Then, when the host calls A.Link(), it will
complete successfully (again by assumption) such that A.[[Status]] =
B.[[Status]] = C.[[Status]] =
linked. These preparatory steps can be performed at any time. Later, when the host is ready to incur any possible side effects of the
modules, it can call A.Evaluate(), which will complete successfully, returning a Promise
resolving to undefined (again by assumption), recursively having evaluated first
C and then B. Each module's [[Status]] at this point will
be evaluated.
Consider then cases involving linking errors, after a successful call to
A.LoadRequestedModules(). If InnerModuleLinking of C succeeds but,
thereafter, fails for B, for example because it imports something that C does not
provide, then the original A.Link() will fail, and both A and B's [[Status]] remain unlinked. C's [[Status]] has become linked, though.
Finally, consider a case involving evaluation errors after a successful call to Link(). If InnerModuleEvaluation of C succeeds but,
thereafter, fails for B, for example because B contains code that throws an
exception, then the original A.Evaluate() will fail, returning a rejected Promise. The
resulting exception will be recorded in both A and B's [[EvaluationError]] fields, and their [[Status]] will
become evaluated. C will also become evaluated
but, in contrast to A and B, will remain without an [[EvaluationError]], as it successfully completed evaluation. Storing the
exception ensures that any time a host tries to reuse A or B by calling their
Evaluate() method, it will encounter the same exception. (Hosts are not required to reuse Cyclic Module Records; similarly,
hosts are not required to expose
the exception objects thrown by these methods. However, the specification enables such uses.)
Now consider a different type of error condition:
Figure 3: A module graph with an unresolvable module
In this scenario, module A declares a dependency on some other module, but no Module
Record exists for that module, i.e. HostLoadImportedModule calls
FinishLoadingImportedModule with an exception
when asked for it. This could occur for a variety of reasons, such as the corresponding resource not
existing, or the resource existing but ParseModule returning some errors when trying to parse the
resulting source text. Hosts can
choose to expose the cause of failure via the completion they pass to FinishLoadingImportedModule. In any case,
this exception causes a loading failure, which results in A's [[Status]] remaining new.
The difference here between loading, linking and evaluation errors is due to the following
characteristic:
Evaluation must
be only performed once, as it can cause side effects; it is thus important to remember whether
evaluation has already been performed, even if unsuccessfully. (In the error case, it makes sense to
also remember the exception because otherwise subsequent Evaluate() calls would have to synthesize a
new one.)
Linking, on the other hand, is side-effect-free, and thus even if it fails, it can be retried at a
later time with no issues.
Loading closely interacts with the host, and it may be desirable for some of them to allow users to retry
failed loads (for example, if the failure is caused by temporarily bad network conditions).
Now, consider a module graph with a cycle:
Figure 4: A cyclic module graph
Here we assume that the entry point is module A, so that the host proceeds by calling
A.LoadRequestedModules(), which performs InnerModuleLoading on A.
This in turn calls InnerModuleLoading on B and C.
Because of the cycle, this again triggers InnerModuleLoading on A, but at this point
it is a no-op since A's dependencies loading has already been triggered during this
LoadRequestedModules process. When all the modules in the graph have been successfully loaded, their
[[Status]] transitions from new to
unlinked at the same time.
Then the host proceeds by calling
A.Link(), which performs InnerModuleLinking on A. This in turn calls
InnerModuleLinking on B. Because of the
cycle, this again triggers InnerModuleLinking on A, but at this point
it is a no-op since A.[[Status]] is already
linking. B.[[Status]] itself remains
linking when control gets back to A and InnerModuleLinking is triggered on C. After
this returns with C.[[Status]] being linked,
both A and B transition from linking to
linked together; this is by design, since they form a strongly connected
component. It's possible to transition the status of modules in the same SCC at the same time because
during this phase the module graph is traversed with a depth-first search.
An analogous story occurs for the evaluation phase of a cyclic module graph, in the success case.
Now consider a case where A has a linking error; for example, it tries to import a binding
from C that does not exist. In that case, the above steps still occur, including the early
return from the second call to InnerModuleLinking on A. However, once we
unwind back to the original InnerModuleLinking on A, it fails during
InitializeEnvironment, namely right after C.ResolveExport(). The thrown
SyntaxError exception propagates up to A.Link, which resets all modules
that are currently on its stack (these are always exactly the modules that are still
linking). Hence both A and B become
unlinked. Note that C is left as linked.
Alternatively, consider a case where A has an evaluation error; for example, its source code
throws an exception. In that case, the evaluation-time analogue of the above steps still occurs,
including the early return from the second call to InnerModuleEvaluation on
A. However, once we unwind back to the original InnerModuleEvaluation on
A, it fails by assumption. The exception thrown propagates up to A.Evaluate(),
which records the error in all modules that are currently on its stack (i.e., the modules
that are still evaluating) as well as via [[AsyncParentModules]], which form a chain for modules which contain or depend on
top-level await through the whole dependency graph through the AsyncModuleExecutionRejected algorithm.
Hence both A and B become evaluated and the exception is
recorded in both A and B's [[EvaluationError]] fields,
while C is left as evaluated with no [[EvaluationError]].
Lastly, consider a module graph with a cycle, where all modules complete asynchronously:
Figure 5: An asynchronous cyclic module graph
Loading and linking happen as before, and all modules end up with [[Status]]
set to linked.
Calling A.Evaluate() calls InnerModuleEvaluation on A,
B, and D, which all transition to evaluating. Then
InnerModuleEvaluation is called on A
again, which is a no-op because it is already evaluating. At this point,
D.[[PendingAsyncDependencies]] is 0, so ExecuteAsyncModule(D) is called and we
call D.ExecuteModule with a new PromiseCapability tracking the asynchronous execution of
D. We unwind back to the InnerModuleEvaluation on B, setting
B.[[PendingAsyncDependencies]] to 1 and B.[[AsyncEvaluationOrder]] to 1. We unwind back to the original InnerModuleEvaluation on A, setting
A.[[PendingAsyncDependencies]] to 1. In the next iteration of the
loop over A's dependencies, we call InnerModuleEvaluation on C and thus on
D (again a no-op) and E. As E has no dependencies and is not part of a
cycle, we call ExecuteAsyncModule(E) in the same manner
as D and E is immediately removed from the stack. We unwind once more to the
InnerModuleEvaluation on C, setting
C.[[AsyncEvaluationOrder]] to 3. Now we finish the loop over
A's dependencies, set A.[[AsyncEvaluationOrder]] to 4,
and remove the entire strongly connected component from the stack, transitioning all of the modules to
evaluating-async at once. At this point, the fields of the modules are as given
in Table 46.
Table 46: Module fields after the initial Evaluate() call
Field
Module
A
B
C
D
E
[[DFSAncestorIndex]]
0
0
0
0
4
[[Status]]
evaluating-async
evaluating-async
evaluating-async
evaluating-async
evaluating-async
[[AsyncEvaluationOrder]]
4
1
3
0
2
[[AsyncParentModules]]
« »
« A »
« A »
« B, C »
« C »
[[PendingAsyncDependencies]]
2 (B and C)
1 (D)
2 (D and E)
0
0
Let us assume that E finishes executing first. When that happens, AsyncModuleExecutionFulfilled is called,
E.[[Status]] is set to evaluated and
C.[[PendingAsyncDependencies]] is decremented to become 1. The
fields of the updated modules are as given in Table 47.
Table 47: Module fields after module E finishes executing
Field
Module
C
E
[[DFSAncestorIndex]]
0
4
[[Status]]
evaluating-async
evaluated
[[AsyncEvaluationOrder]]
3
done
[[AsyncParentModules]]
« A »
« C »
[[PendingAsyncDependencies]]
1 (D)
0
D is next to finish (as it was the only module that was still executing). When that happens,
AsyncModuleExecutionFulfilled is called
again and D.[[Status]] is set to evaluated.
Its ancestors available for execution are B (whose [[AsyncEvaluationOrder]] is 1) and C (whose [[AsyncEvaluationOrder]] is 3), thus B will be handled first:
B.[[PendingAsyncDependencies]] is decremented to become 0, ExecuteAsyncModule is called on B, and it
starts executing. C.[[PendingAsyncDependencies]] is also decremented
to become 0, and C starts executing (potentially in parallel to B if B
contains an await). The fields of the updated modules are as given in Table 48.
Table 48: Module fields after module D finishes executing
Field
Module
B
C
D
[[DFSAncestorIndex]]
0
0
0
[[Status]]
evaluating-async
evaluating-async
evaluated
[[AsyncEvaluationOrder]]
1
3
done
[[AsyncParentModules]]
« A »
« A »
« B, C »
[[PendingAsyncDependencies]]
0
0
0
Let us assume that C finishes executing next. When that happens, AsyncModuleExecutionFulfilled is called
again, C.[[Status]] is set to evaluated and
A.[[PendingAsyncDependencies]] is decremented to become 1. The
fields of the updated modules are as given in Table 49.
Table 49: Module fields after module C finishes executing
Field
Module
A
C
[[DFSAncestorIndex]]
0
0
[[Status]]
evaluating-async
evaluated
[[AsyncEvaluationOrder]]
4
done
[[AsyncParentModules]]
« »
« A »
[[PendingAsyncDependencies]]
1 (B)
0
Then, B finishes executing. When that happens, AsyncModuleExecutionFulfilled is called
again and B.[[Status]] is set to evaluated.
A.[[PendingAsyncDependencies]] is decremented to become 0, so
ExecuteAsyncModule is called and it starts
executing. The fields of the updated modules are as given in Table 50.
Table 50: Module fields after module B finishes executing
Field
Module
A
B
[[DFSAncestorIndex]]
0
0
[[Status]]
evaluating-async
evaluated
[[AsyncEvaluationOrder]]
4
done
[[AsyncParentModules]]
« »
« A »
[[PendingAsyncDependencies]]
0
0
Finally, A finishes executing. When that happens, AsyncModuleExecutionFulfilled is called
again and A.[[Status]] is set to evaluated.
At this point, the Promise in A.[[TopLevelCapability]] (which was
returned from A.Evaluate()) is resolved, and this concludes the handling of this module
graph. The fields of the updated module are as given in Table 51.
Table 51: Module fields after module A finishes executing
Field
Module
A
[[DFSAncestorIndex]]
0
[[Status]]
evaluated
[[AsyncEvaluationOrder]]
done
[[AsyncParentModules]]
« »
[[PendingAsyncDependencies]]
0
Alternatively, consider a failure case where C fails execution and returns an error before
B has finished executing. When that happens, AsyncModuleExecutionRejected is called,
which sets C.[[Status]] to evaluated and
C.[[EvaluationError]] to the error. It then propagates this error to
all of the AsyncParentModules by performing AsyncModuleExecutionRejected on each of
them. The fields of the updated modules are as given in Table 52.
Table 52: Module fields after module C finishes with an error
Field
Module
A
C
[[DFSAncestorIndex]]
0
0
[[Status]]
evaluated
evaluated
[[AsyncEvaluationOrder]]
done
done
[[AsyncParentModules]]
« »
« A »
[[PendingAsyncDependencies]]
1 (B)
0
[[EvaluationError]]
empty
C's evaluation error
A will be rejected with the same error as C since C will call
AsyncModuleExecutionRejected on
A with C's error. A.[[Status]] is set to
evaluated. At this point the Promise in A.[[TopLevelCapability]] (which was returned from A.Evaluate()) is
rejected. The fields of the updated module are as given in Table 53.
Table 53: Module fields after module A is rejected
Then, B finishes executing without an error. When that happens, AsyncModuleExecutionFulfilled is called
again and B.[[Status]] is set to evaluated.
GatherAvailableAncestors is called on
B. However, A.[[CycleRoot]] is A which has an
evaluation error, so it will not be added to the returned sortedExecList and AsyncModuleExecutionFulfilled will
return without further processing. Any future importer of B will resolve the rejection of
B.[[CycleRoot]].[[EvaluationError]] from
the evaluation error from C that was set on the cycle root A. The fields of the
updated modules are as given in Table 54.
Table 54: Module fields after module B finishes executing in an erroring graph
A Source Text Module
Record is used to represent information about a module that was defined from ECMAScript source text
(11) that was parsed using the goal
symbolModule. Its fields
contain digested information about the names that are imported and exported by the module, and its
concrete methods use these digests to link and evaluate the module.
A List of ExportEntry records
derived from the code of this module that correspond to reexported imports that occur within the
module or exports from export * as namespace declarations.
A List of ExportEntry records
derived from the code of this module that correspond to export * declarations that
occur within the module, not including export * as namespace declarations.
An ImportEntry Record is
a Record that digests information about a
single declarative import. Each ImportEntry Record has the fields defined in Table
56:
The name under which the desired binding is exported by the module identified by [[ModuleRequest]]. The value namespace-object
indicates that the import request is for the target module's namespace object.
[[LocalName]]
a String
The name that is used to locally access the imported value from within the importing module.
Note 1
Table 57 gives examples
of ImportEntry records fields used to represent the syntactic import forms:
An ExportEntry Record is
a Record that digests information about a
single declarative export. Each ExportEntry Record has the fields defined in Table
58:
The name under which the desired binding is exported by the module identified by [[ModuleRequest]]. null if the ExportDeclaration does not have
a ModuleSpecifier.
all is used for export * as ns from "mod" declarations.
all-but-default is used for export * from "mod"
declarations.
[[LocalName]]
a String or null
The name that is used to locally access the exported value from within the importing module.
null if the exported value is not locally accessible from within the module.
Note 2
Table 59 gives examples
of the ExportEntry record fields used to represent the syntactic export forms:
An implementation may parse module source text and analyse it for Early Error conditions prior to
the evaluation of ParseModule for that module source text. However, the reporting of any errors must
be deferred until the point where this specification actually performs ParseModule upon that source
text.
16.2.1.7.2 Implementation of Module Record Abstract Methods
The ResolveExport concrete method of a Source Text Module Recordmodule takes
argument exportName (a String) and optional argument resolveSet (a List of Records with fields [[Module]] (a Module Record) and [[ExportName]] (a String)) and returns a ResolvedBinding
Record, null, or ambiguous.
ResolveExport attempts to resolve an imported binding to the actual defining module and local binding
name. The defining module may be the module represented by the Module
Record this method was invoked on or some other module that is imported by that
module. The parameter resolveSet is used to detect unresolved circular import/export paths.
If a pair consisting of specific Module Record and exportName is
reached that is already in resolveSet, an import circularity has been encountered. Before
recursively calling ResolveExport, a pair consisting of module and exportName is
added to resolveSet.
If a defining module is found, a ResolvedBinding Record { [[Module]], [[BindingName]] } is returned. This record
identifies the resolved binding of the originally requested export, unless this is the export of a
namespace with no local binding. In this case, [[BindingName]] will be set to
namespace. If no definition was found or the request is found to be circular,
null is returned. If the request is found to be ambiguous,
ambiguous is returned.
Assert:
There is more than one * import that includes the requested name.
If resolution.[[Module]] and
starResolution.[[Module]] are not the same
Module Record, return
ambiguous.
If resolution.[[BindingName]] is not
starResolution.[[BindingName]] and either
resolution.[[BindingName]] or
starResolution.[[BindingName]] is
namespace, return ambiguous.
If resolution.[[BindingName]]is a String,
starResolution.[[BindingName]]is a String, and
resolution.[[BindingName]] is not
starResolution.[[BindingName]], return
ambiguous.
Return starResolution.
16.2.1.7.3 Implementation of Cyclic Module Record Abstract Methods
A Synthetic Module Record is used to
represent information about a module that is defined by specifications. Its exported names are statically
defined at creation, while their corresponding values can change over time using SetSyntheticModuleExport. It has no imports or
dependencies.
Note
A Synthetic Module Record could be used for defining a variety of module types:
for example, JSON modules or CSS modules.
In addition to the fields defined in Table 41 Synthetic Module Records have the
additional fields listed in Table 60.
The initialization logic to perform upon evaluation of the module, taking the Synthetic Module Record as its sole
argument. It must not modify [[ExportNames]]. It may return an
abrupt completion.
The abstract operation SetSyntheticModuleExport takes arguments module (a Synthetic
Module Record), exportName (a String), and exportValue (an
ECMAScript language value) and returns
unused. It can be used to set or change the exported value for an existing export
of a Synthetic Module Record. It performs the
following steps when called:
The LoadRequestedModules concrete method of a Synthetic Module Recordmodule takes no arguments and returns a Promise. It performs the following steps when
called:
The GetExportedNames concrete method of a Synthetic Module Recordmodule takes no arguments and returns a List of Strings. It performs the
following steps when called:
Return module.[[ExportNames]].
16.2.1.8.4.3 ResolveExport ( exportName )
The ResolveExport concrete method of a Synthetic Module Recordmodule takes argument exportName (a String) and returns a ResolvedBinding
Record or null. It performs the following steps when called:
If module.[[ExportNames]] does not contain
exportName, return null.
The actual process performed is host-defined, but typically consists of performing whatever I/O
operations are necessary to load the appropriate Module Record. Multiple different
(referrer, moduleRequest.[[Specifier]],
moduleRequest.[[Attributes]]) triples may map to the same Module
Record instance. The actual mapping semantics is host-defined but typically a normalization process
is applied to specifier as part of the mapping process. A typical normalization process would
include actions such as expansion of relative and abbreviated path specifiers.
Note 2
The above text requires that hosts support JSON modules when imported with
type: "json" (and HostLoadImportedModule completes normally), but it does not prohibit
hosts from supporting JSON
modules when imported without type: "json".
16.2.1.11 FinishLoadingImportedModule ( referrer,
moduleRequest, payload, result )
Append the LoadedModuleRequest Record { [[Specifier]]: moduleRequest.[[Specifier]], [[Attributes]]:
moduleRequest.[[Attributes]], [[Module]]: result.[[Value]] }
to referrer.[[LoadedModules]].
The abstract operation AllImportAttributesSupported takes argument attributes (a List of ImportAttribute
Records) and returns a Boolean. It performs the following steps when called:
If supported does not contain attribute.[[Key]],
return false.
Return true.
16.2.1.12.1 HostGetSupportedImportAttributes ( )
The host-defined
abstract operation HostGetSupportedImportAttributes takes no arguments and returns a List of Strings. It allows host environments
to specify which import attributes they support. Only attributes with supported keys will be provided to
the host.
An implementation of HostGetSupportedImportAttributes must conform to the following requirements:
It must return a List of Strings, each indicating a
supported attribute.
Each time this operation is called, it must return the same List with the same contents in the
same order.
The default implementation of HostGetSupportedImportAttributes is to return a new empty List.
Note
The purpose of requiring the host to specify its supported import attributes, rather than passing
all attributes to the host and
letting it then choose which ones it wants to handle, is to ensure that unsupported attributes are
handled in a consistent way across different hosts.
16.2.1.13 GetModuleNamespace ( module )
The abstract operation GetModuleNamespace takes argument module (an instance of a concrete
subclass of Module Record) and returns a Module Namespace
Object. It retrieves the Module Namespace Object representing module's exports, lazily creating
it the first time it was requested, and storing it in module.[[Namespace]] for future retrieval. It performs the following steps when called:
GetModuleNamespace never throws. Instead, unresolvable names are simply excluded from the namespace
at this point. They will lead to a real linking error later unless they are all ambiguous star exports
that are not explicitly requested anywhere.
Sort attributes according to the lexicographic order of their [[Key]] field, treating the value of each such field as a sequence of UTF-16
code unit values. NOTE: This sorting is observable only in that hosts are prohibited from changing behaviour based on
the order in which attributes are enumerated.
Return a List whose sole element is a new
ExportEntry
Record { [[ModuleRequest]]: null, [[ImportName]]: null, [[LocalName]]:
localName, [[ExportName]]: "default" }.
Return a List whose sole element is a new
ExportEntry
Record { [[ModuleRequest]]: null, [[ImportName]]: null, [[LocalName]]:
localName, [[ExportName]]: "default" }.
Return a List whose sole element is a new
ExportEntry
Record { [[ModuleRequest]]: module, [[ImportName]]: importName, [[LocalName]]:
localName, [[ExportName]]: sourceName }.
Return a List whose sole element is a new
ExportEntry
Record { [[ModuleRequest]]: module, [[ImportName]]: importName, [[LocalName]]:
localName, [[ExportName]]: exportName }.
An implementation must report most errors at the time the relevant ECMAScript language construct is evaluated.
An early error is an error that can be
detected and reported prior to the evaluation of any construct in the Script containing the error. The presence of an early error prevents the evaluation
of the construct. An implementation must report early errors in a Script as part of parsing that Script in ParseScript. Early errors in a Module are reported at the point when the Module would be evaluated and the Module is never initialized. Early errors in eval code are reported at the time
eval is called and prevent evaluation of the eval code. All errors that are not early errors are runtime errors.
An implementation must report as an early
error any occurrence of a condition that is listed in a “Static Semantics: Early Errors”
subclause of this specification.
An implementation shall not treat other kinds of errors as early errors even if the compiler can prove that a construct cannot
execute without error under any circumstances. An implementation may issue an early warning in such a case, but
it should not report the error until the relevant construct is actually executed.
An implementation shall report all errors as specified, except for the following:
Except as restricted in 17.1, a host or implementation may extend Script syntax, Module syntax, and regular expression pattern or flag syntax. To permit
this, all operations (such as calling eval, using a regular expression literal, or using the
Function or RegExp constructor) that are allowed to throw SyntaxError
are permitted to exhibit host-defined behaviour instead of throwing
SyntaxError when they encounter a host-defined extension to the script syntax or regular expression
pattern or flag syntax.
Except as restricted in 17.1, a host or implementation may provide additional types, values, objects,
properties, and functions beyond those described in this specification. This may cause constructs (such as
looking up a variable in the global scope) to have host-defined behaviour instead of throwing an error (such as
ReferenceError).
17.1 Forbidden Extensions
An implementation must not extend this specification in the following ways:
If an implementation extends any function object with an own property named
"caller" the value of that property, as observed using [[Get]]
or [[GetOwnProperty]], must not be a strict function object. If it is an accessor property, the
function that is the value of the property's [[Get]] attribute must never return a
strict function
when called.
Neither mapped nor unmapped arguments objects may be created with an own property named
"caller".
The behaviour of built-in methods which are specified in ECMA-402, such as those named
toLocaleString, must not be extended except as specified in ECMA-402.
The RegExp pattern grammars in 22.2.1 and B.1.2 must not be extended to
recognize any of the source characters A-Z or a-z as IdentityEscape[+UnicodeMode]
when the [UnicodeMode] grammar parameter is present.
The Syntactic Grammar must not be extended in any manner that allows the token : to immediately
follow source text that is matched by the BindingIdentifier nonterminal symbol.
There are certain built-in objects available whenever an ECMAScript Script or Module begins execution. One, the global object, is part of the global environment of
the executing program. Others are accessible as initial properties of the global object or indirectly as properties of
accessible built-in objects.
Unless specified otherwise, a built-in object that is callable as a function is a built-in function object with the
characteristics described in 10.3. Unless specified otherwise, the [[Extensible]] internal slot of a built-in object initially has the value
true. Every built-in function object has a [[Realm]] internal
slot whose value is the Realm
Record of the realm
for which the object was initially created.
Many built-in objects are functions: they can be invoked with arguments. Some of them furthermore are constructors: they are functions
intended for use with the new operator. For each built-in function, this specification describes
the arguments required by that function and the properties of that function object. For each built-in constructor, this specification
furthermore describes properties of the prototype object of that constructor and properties of specific object instances returned by a
new expression that invokes that constructor.
Unless otherwise specified in the description of a particular function, if a built-in function or constructor is given fewer arguments
than the function is specified to require, the function or constructor shall behave exactly as if it had been given sufficient
additional arguments, each such argument being the undefined value. Such missing arguments
are considered to be “not present” and may be identified in that manner by specification algorithms. In the
description of a particular function, the terms “this value” and “NewTarget” have the
meanings given in 10.3.
Unless otherwise specified in the description of a particular function, if a built-in function or constructor described is given more
arguments than the function is specified to allow, the extra arguments are evaluated by the call and then
ignored by the function. However, an implementation may define implementation specific behaviour relating to
such arguments as long as the behaviour is not the throwing of a TypeError exception that is
predicated simply on the presence of an extra argument.
Note 1
Implementations that add additional capabilities to the set of built-in functions are encouraged to do so
by adding new functions rather than adding new parameters to existing functions.
Unless otherwise specified every built-in function and every built-in constructor has the Function prototype object, which is
the initial value of the expression Function.prototype (20.2.3), as the value of its [[Prototype]] internal slot.
Unless otherwise specified every built-in prototype object has the Object prototype object, which is the
initial value of the expression Object.prototype (20.1.3), as the value of its [[Prototype]] internal slot, except the Object prototype object itself.
If this specification defines a built-in constructor's behaviour via algorithm steps, then that is its behaviour
for the purposes of both [[Call]] and [[Construct]]. If such
an algorithm needs to distinguish the two cases, it checks whether NewTarget is undefined,
which indicates a [[Call]] invocation.
Built-in function
objects that are not identified as constructors do not implement the [[Construct]]
internal method unless otherwise specified in the description of a particular function.
Built-in function
objects that are not constructors do not have a "prototype" property unless
otherwise specified in the description of a particular function.
Each built-in function defined in this specification is created by calling the CreateBuiltinFunction abstract operation (10.3.4).
The values of the length and name parameters are the initial values of the
"length" and "name" properties as discussed below. The values of the
prefix parameter are similarly discussed below.
Every built-in function
object, including constructors, has a "length" property whose value is a
non-negative integral
Number. Unless otherwise specified, this value is the number of required parameters shown in
the subclause heading for the function description. Optional parameters and rest parameters are not included in
the parameter count.
Note 2
For example, the function
object that is the initial value of the "map" property of the
Array prototype object is described
under the subclause heading «Array.prototype.map (callback [ , thisArg])» which shows the two named
arguments callback and thisArg, the latter being optional; therefore the value of the
"length" property of that function object is 1𝔽.
Unless otherwise specified, the "length" property of a built-in function object has the
attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true }.
Every built-in function
object, including constructors, has a "name" property whose value
is a String. Unless otherwise specified,
this value is the name that is given to the function in this specification. Functions that are identified as
anonymous functions use the empty String as the value of the "name" property. For functions
that are specified as properties of objects, the name value is the property name string used to access the function.
Functions that are specified as get or set accessor functions of built-in properties have
"get" or "set" (respectively) passed to the prefix parameter
when calling CreateBuiltinFunction.
The value of the "name" property is explicitly specified for each built-in functions whose
property keyis a Symbol value. If such an explicitly
specified value starts with the prefix "get " or "set " and the function
for which it is specified is a get or set accessor function of a built-in property, the value without the prefix
is passed to the name parameter, and the value "get" or "set"
(respectively) is passed to the prefix parameter when calling CreateBuiltinFunction.
Unless otherwise specified, the "name" property of a built-in function object has the
attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true }.
Every other data
property described in clauses 19 through 28 and in Annex B.2 has the attributes { [[Writable]]: true, [[Enumerable]]:
false, [[Configurable]]: true } unless otherwise
specified.
Every accessor
property described in clauses 19 through 28 and in Annex B.2 has the attributes { [[Enumerable]]: false, [[Configurable]]:
true } unless otherwise specified. If only a get accessor function is described, the set
accessor function is the default value, undefined. If only a set accessor is described the
get accessor is the default value, undefined.
does not have a [[Construct]] internal method; it cannot be used as a constructor with the
new operator.
does not have a [[Call]] internal method; it cannot be invoked as a function.
has a [[Prototype]] internal slot whose value is host-defined.
may have host-defined
properties in addition to the properties defined in this specification. This may include a property whose
value is the global object itself.
19.1 Value Properties of the Global Object
19.1.1 globalThis
The initial value of the "globalThis" property of the global object in a
Realm Recordrealm is realm.[[GlobalEnv]].[[GlobalThisValue]].
This property has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]:
true }.
19.1.2 Infinity
The value of Infinity is +∞𝔽 (see 6.1.6.1). This property has the
attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
19.1.3 NaN
The value of NaN is NaN (see 6.1.6.1). This property has the
attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
19.1.4 undefined
The value of undefined is undefined (see 6.1.1). This property has the
attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
NOTE: If direct is true, runningContext will be the
execution
context that performed the direct eval. If
direct is false, runningContext will be the execution
context for the invocation of the eval function.
The eval code cannot instantiate variable or function bindings in the variable environment of the
calling context that invoked the eval if either the code of the calling context or the eval code is
strict mode
code. Instead such bindings are instantiated in a new VariableEnvironment that is
only accessible to the eval code. Bindings introduced by let, const, or
class declarations are always instantiated in a new LexicalEnvironment.
19.2.1.2 HostEnsureCanCompileStrings ( calleeRealm,
parameterStrings, bodyString, direct )
The host-defined
abstract operation HostEnsureCanCompileStrings takes arguments calleeRealm (a Realm Record),
parameterStrings (a List of Strings), bodyString
(a String), and direct (a Boolean) and returns either a normal completion containingunused or a throw completion. It
allows host
environments to block certain ECMAScript functions which allow developers to interpret
and evaluate strings as ECMAScript code.
parameterStrings represents the strings that, when using one of the function constructors, will be
concatenated together to build the parameters list. bodyString represents the function body or
the string passed to an eval call.
direct signifies whether the evaluation is a direct eval.
The default implementation of HostEnsureCanCompileStrings is to return NormalCompletion(unused).
Let trimmedPrefix be the longest prefix of trimmed that satisfies the syntax of
a StrDecimalLiteral, which might
be trimmed itself. If there is no such prefix, return NaN.
This function may interpret only a leading portion of string as a Number value; it ignores
any code units that cannot be interpreted as part of the notation of a decimal literal, and no
indication is given that any such code units were ignored.
19.2.5 parseInt ( string, radix )
This function produces an integral Number dictated by interpretation of the contents of
string according to the specified radix. Leading white space in string is
ignored. If radix coerces to 0 (such as when it is undefined), it is assumed
to be 10 except when the number representation begins with "0x" or
"0X", in which case it is assumed to be 16. If radix is 16, the number
representation may optionally begin with "0x" or "0X".
If S is not empty and the first code unit of S is the code unit 0x002D
(HYPHEN-MINUS), set sign to -1.
If S is not empty and the first code unit of S is either the code unit 0x002B
(PLUS SIGN) or the code unit 0x002D (HYPHEN-MINUS), set S to the substring of S from index 1.
If S contains a code unit that is not a radix-R digit, let end be the
index within S of the first such code unit; otherwise let end be the length of
S.
Let mathInt be the integer value that is represented by Z in
radix-R notation, using the letters A through Z and a through z
for digits with values 10 through 35. (However, if R = 10 and Z contains more than
20 significant digits, every significant digit after the 20th may be replaced by a 0 digit, at the
option of the implementation; and if R is not one of 2, 4, 8, 10, 16, or 32, then
mathInt may be an implementation-approximatedinteger representing the integer value denoted by Z
in radix-R notation.)
This function may interpret only a leading portion of string as an integer value; it ignores any code units that cannot
be interpreted as part of the notation of an integer, and no indication is given that any such code units were
ignored.
19.2.6 URI Handling Functions
Uniform Resource Identifiers, or URIs, are Strings that identify resources (e.g. web pages or files) and
transport protocols by which to access them (e.g. HTTP or FTP) on the Internet. The ECMAScript language
itself does not provide any support for using URIs except for functions that encode and decode URIs as
described in this section. encodeURI and decodeURI are intended to work with
complete URIs; they assume that any reserved characters are intended to have special meaning (e.g., as
delimiters) and so are not encoded. encodeURIComponent and decodeURIComponent are
intended to work with the individual components of a URI; they assume that any reserved characters represent
text and must be encoded to avoid special meaning when the component is part of a complete URI.
Note 1
The set of reserved characters is based upon RFC 2396 and does not reflect changes introduced by the
more recent RFC 3986.
Note 2
Many implementations of ECMAScript provide additional functions and methods that manipulate web pages;
these functions are beyond the scope of this standard.
19.2.6.1 decodeURI ( encodedURI )
This function computes a new version of a URI in which each escape sequence and UTF-8 encoding of the
sort that might be introduced by the encodeURI function is replaced with the UTF-16 encoding
of the code point that it represents. Escape sequences that could not have been introduced by
encodeURI are not replaced.
This function computes a new version of a URI in which each escape sequence and UTF-8 encoding of the
sort that might be introduced by the encodeURIComponent function is replaced with the UTF-16
encoding of the code point that it represents.
It is the %decodeURIComponent% intrinsic object.
It performs the following steps when called:
Let componentString be ? ToString(encodedURIComponent).
This function computes a new version of a UTF-16 encoded (6.1.4) URI in which each instance of
certain code points is replaced by one, two, three, or four escape sequences representing the UTF-8
encoding of the code point.
This function computes a new version of a UTF-16 encoded (6.1.4) URI in which each instance of
certain code points is replaced by one, two, three, or four escape sequences representing the UTF-8
encoding of the code point.
The abstract operation Encode takes arguments string (a String) and extraUnescaped
(a String) and returns either a normal completion
containing a String or a throw completion. It
performs URI encoding and escaping, interpreting string as a sequence of UTF-16 encoded code
points as described in 6.1.4. If a character is identified
as unreserved in RFC 2396 or appears in extraUnescaped, it is not escaped. It performs the
following steps when called:
Because percent-encoding is used to represent individual octets, a single code point may be expressed
as multiple consecutive escape sequences (one for each of its 8-bit UTF-8 code units).
19.2.6.6 Decode ( string, preserveEscapeSet )
The abstract operation Decode takes arguments string (a String) and
preserveEscapeSet (a String) and returns either a normal completion containing a String
or a throw completion. It performs URI
unescaping and decoding, preserving any escape sequences that correspond to Basic Latin characters in
preserveEscapeSet. It performs the following steps when called:
Let len be the length of string.
Let R be the empty String.
Let k be 0.
Repeat, while k < len,
Let C be the code unit at index k within string.
Let S be C.
If C is the code unit 0x0025 (PERCENT SIGN), then
If k + 3 > len, throw a URIError exception.
Let escape be the substring of string from k to
k + 3.
RFC 3629 prohibits the decoding of invalid UTF-8 octet sequences. For example, the invalid sequence
0xC0 0x80 must not decode into the code unit 0x0000. Implementations of the Decode algorithm are
required to throw a URIError when encountering such invalid sequences.
19.2.6.7 ParseHexOctet ( string, position )
The abstract operation ParseHexOctet takes arguments string (a String) and position
(a non-negative integer) and
returns either a non-negative integer or a non-empty List of SyntaxError
objects. It parses a sequence of two hexadecimal characters at the specified position in
string into an unsigned 8-bit integer. It performs the following steps when called:
callback should be a function that accepts two arguments. groupBy calls
callback once for each element in items, in ascending order, and constructs a
new object. Each value returned by callback is coerced to a property key. For each such property key, the result
object has a property whose key is that property key and whose value is an array containing all the
elements for which the callback return value coerced to that key.
callback is called with two arguments: the value of the element and the index of the
element.
The return value of groupBy is an object that does not inherit from %Object.prototype%.
This function performs the following steps when called:
Let groups be ? GroupBy(items, callback,
property).
The ordering of steps 1 and 2 is chosen to ensure that any exception that
would have been thrown by step 1 in previous editions of this
specification will continue to be thrown even if the this value is
undefined or null.
20.1.3.3 Object.prototype.isPrototypeOf ( V )
This method performs the following steps when called:
The ordering of steps 1 and 2 preserves the behaviour specified by previous
editions of this specification for the case where V is not an object and the
this value is undefined or null.
20.1.3.4 Object.prototype.propertyIsEnumerable ( V )
This method performs the following steps when called:
This method does not consider objects in the prototype chain.
Note 2
The ordering of steps 1 and 2 is chosen to ensure that any exception
that would have been thrown by step 1 in previous
editions of this specification will continue to be thrown even if the this value is
undefined or null.
The optional parameters to this method are not used but are intended to correspond to the parameter
pattern used by ECMA-402 toLocaleString methods. Implementations that do not include ECMA-402
support must not use those parameter positions for other purposes.
Note 1
This method provides a generic toLocaleString implementation for objects that have no
locale-sensitive toString behaviour. Array, Number,
Date, and %TypedArray% provide their own
locale-sensitive toLocaleString methods.
Note 2
ECMA-402 intentionally does not provide an alternative to this default implementation.
20.1.3.6 Object.prototype.toString ( )
This method performs the following steps when called:
If the this value is undefined, return "[object
Undefined]".
If the this value is null, return "[object
Null]".
Historically, this method was occasionally used to access the String value of the [[Class]] internal slot that was used in previous editions of this specification
as a nominal type tag for various built-in objects. The above definition of toString
preserves compatibility for legacy code that uses toString as a test for those specific
kinds of built-in objects. It does not provide a reliable type testing mechanism for other kinds of
built-in or program defined objects. In addition, programs can use %Symbol.toStringTag% in ways that will invalidate
the reliability of such legacy type tests.
20.1.3.7 Object.prototype.valueOf ( )
This method performs the following steps when called:
Object.prototype.__proto__ is an accessor property with attributes { [[Enumerable]]: false, [[Configurable]]:
true }. The [[Get]] and [[Set]]
attributes are defined as follows:
20.1.3.8.1 get Object.prototype.__proto__
The value of the [[Get]] attribute is a built-in function that requires no
arguments. It performs the following steps when called:
is the initial value of the "Function" property of the global object.
creates and initializes a new function object when called as a function rather than as a
constructor. Thus the
function call Function(…) is equivalent to the object creation expression
new Function(…) with the same arguments.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to
inherit the specified Function behaviour must include a super call to the Function constructor to create and
initialize a subclass instance with the internal slots necessary for built-in function behaviour. All
ECMAScript syntactic forms for defining function objects create instances of Function. There is no
syntactic means to create instances of Function subclasses except for the built-in GeneratorFunction,
AsyncFunction, and AsyncGeneratorFunction subclasses.
20.2.1.1 Function ( ...parameterArgs, bodyArg )
The last argument (if any) specifies the body (executable code) of a function; any preceding arguments
specify formal parameters.
This function performs the following steps when called:
It is permissible but not necessary to have one argument for each formal parameter to be specified.
For example, all three of the following expressions produce the same result:
The abstract operation CreateDynamicFunction takes arguments constructor (a constructor),
newTarget (a constructor or undefined), kind
(normal, generator, async, or
async-generator), parameterArgs (a List of ECMAScript language values), and
bodyArg (an ECMAScript language value) and returns either a
normal completion containing an
ECMAScript function
object or a throw completion.
constructor is the constructor function that is performing this action.
newTarget is the constructor that new was initially applied to.
parameterArgs and bodyArg reflect the argument values that were passed to
constructor. It performs the following steps when called:
If newTarget is undefined, set newTarget to
constructor.
If body is a List of errors, throw a
SyntaxError exception.
NOTE: The parameters and body are parsed separately to ensure that each is valid alone. For
example, new Function("/*", "*/ ) {") does not evaluate to a function.
NOTE: If this step is reached, sourceText must have the syntax of exprSym
(although the reverse implication does not hold). The purpose of the next two steps is to enforce
any Early Error rules which apply to exprSym directly.
NOTE: Functions whose kind is async are not constructable and do
not have a [[Construct]] internal method or a
"prototype" property.
Return F.
Note
CreateDynamicFunction defines a "prototype" property on any function it creates
whose kind is not async to provide for the possibility that the
function will be used as a constructor.
has a "name" property whose value is the empty String.
Note
The Function prototype object is specified to be a function object to ensure compatibility with ECMAScript code
that was created prior to the ECMAScript 2015 specification.
The thisArg value is passed without modification as the this value.
This is a change from Edition 3, where an undefined or nullthisArg is replaced with the global object and ToObject is applied to all other values and
that result is passed as the this value. Even though the thisArg is
passed without modification, non-strict functions still perform these
transformations upon entry to the function.
Note 2
If func is either an arrow function or a bound function exotic object,
then the thisArg will be ignored by the function [[Call]] in step
6.
Function
objects created using Function.prototype.bind are exotic objects. They
also do not have a "prototype" property.
Note 2
If Target is either an arrow function or a bound function exotic object,
then the thisArg passed to this method will not be used by subsequent calls to
F.
The thisArg value is passed without modification as the this value.
This is a change from Edition 3, where an undefined or nullthisArg is replaced with the global object and ToObject is applied to all other values and
that result is passed as the this value. Even though the thisArg is
passed without modification, non-strict functions still perform these
transformations upon entry to the function.
Note 2
If func is either an arrow function or a bound function exotic object,
then the thisArg will be ignored by the function [[Call]] in step
4.
20.2.3.4 Function.prototype.constructor
The initial value of Function.prototype.constructor is %Function%.
20.2.3.5 Function.prototype.toString ( )
This method performs the following steps when called:
Let func be the this value.
If funcis an
Object, func has a [[SourceText]] internal
slot, func.[[SourceText]] is a sequence of Unicode code points,
and HostHasSourceTextAvailable(func)
is true, then
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
Note
This is the default implementation of %Symbol.hasInstance% that most functions inherit.
%Symbol.hasInstance% is called by the instanceof operator to determine
whether a value is an instance of a specific constructor. An expression such as
v instanceof F
evaluates as
F[%Symbol.hasInstance%](v)
A constructor
function can control which objects are recognized as its instances by instanceof by
exposing a different %Symbol.hasInstance% method on the function.
This property is non-writable and non-configurable to prevent tampering that could be used to globally
expose the target function of a bound function.
The value of the "name" property of this method is
"[Symbol.hasInstance]".
20.2.4 Function Instances
Every Function instance is an ECMAScript function object and has the internal slots listed in Table 28. Function objects created
using the Function.prototype.bind method (20.2.3.2) have the internal slots
listed in Table 29.
Function instances have the following properties:
20.2.4.1 length
The value of the "length" property is an integral Number that indicates the typical
number of arguments expected by the function. However, the language permits the function to be invoked
with some other number of arguments. The behaviour of a function when invoked on a number of arguments
other than the number specified by its "length" property depends on the function. This
property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true }.
20.2.4.2 name
The value of the "name" property is a String that is descriptive of
the function. The name has no semantic significance but is typically a variable or property name that is used
to refer to the function at its point of definition in ECMAScript source text. This property has the attributes { [[Writable]]: false, [[Enumerable]]:
false, [[Configurable]]: true }.
Anonymous functions objects that do not have a contextual name associated with them by this specification
use the empty String as the value of the "name" property.
20.2.4.3 prototype
Function instances that can be used as a constructor have a "prototype" property.
Whenever such a Function instance is created another ordinary object is also created and is the initial value of the
function's "prototype" property. Unless otherwise specified, the value of the
"prototype" property is used to initialize the [[Prototype]]
internal slot of the object created when that function is invoked as a constructor.
This property has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]:
false }.
The host-defined
abstract operation HostHasSourceTextAvailable takes argument func (a function object) and
returns a Boolean. It allows host environments to prevent the source text from being provided
for func.
An implementation of HostHasSourceTextAvailable must conform to the following requirements:
It must be deterministic with respect to its parameters. Each time it is called with a specific
func as its argument, it must return the same result.
The default implementation of HostHasSourceTextAvailable is to return true.
is the initial value of the "Boolean" property of the global object.
creates and initializes a new Boolean object when called as a constructor.
performs a type conversion when called as a function rather than as a constructor.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to
inherit the specified Boolean behaviour must include a super call to the Boolean constructor to create and
initialize the subclass instance with a [[BooleanData]] internal slot.
20.3.1.1 Boolean ( value )
This function performs the following steps when called:
Boolean instances are ordinary
objects that inherit properties from the Boolean prototype object. Boolean
instances have a [[BooleanData]] internal slot. The [[BooleanData]] internal slot is the Boolean value represented by this Boolean object.
The GlobalSymbolRegistry List is an append-only List that is globally available. It is
shared by all realms. Prior to the
evaluation of any ECMAScript code, it is initialized as a new empty List. Elements of the
GlobalSymbolRegistry List are Records with the structure defined in
Table 61.
The initial value of Symbol.prototype.constructor is %Symbol%.
20.4.3.2 get Symbol.prototype.description
Symbol.prototype.description is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
The initial value of the %Symbol.toStringTag% property is the String value
"Symbol".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true }.
20.4.4 Properties of Symbol Instances
Symbol instances are ordinary
objects that inherit properties from the Symbol prototype object. Symbol
instances have a [[SymbolData]] internal slot. The [[SymbolData]] internal slot is the Symbol value represented by this Symbol object.
20.4.5 Abstract Operations for Symbols
20.4.5.1 KeyForSymbol ( sym )
The abstract operation KeyForSymbol takes argument sym (a Symbol) and returns a String or
undefined. If sym is in the GlobalSymbolRegistry List, the String used to
register sym will be returned. It performs the following steps when called:
Instances of Error objects are thrown as exceptions when runtime errors occur. The Error objects may also
serve as base objects for user-defined exception classes.
When an ECMAScript implementation detects a runtime error, it throws a new instance of one of the
NativeError objects defined in 20.5.5 or a new
instance of the AggregateError object defined in 20.5.7.
is the initial value of the "Error" property of the global object.
creates and initializes a new Error object when called as a function rather than as a constructor. Thus the function
call Error(…) is equivalent to the object creation expression new Error(…) with
the same arguments.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to
inherit the specified Error behaviour must include a super call to the Error constructor to create and
initialize subclass instances with an [[ErrorData]] internal slot.
20.5.1.1 Error ( message [ , options ] )
This function performs the following steps when called:
If NewTarget is undefined, let newTarget be the active function
object; else let newTarget be NewTarget.
If msg is undefined, set msg to the empty String; otherwise
set msg to ? ToString(msg).
If name is the empty String, return msg.
If msg is the empty String, return name.
Return the string-concatenation of name, the code unit
0x003A (COLON), the code unit 0x0020 (SPACE), and msg.
20.5.4 Properties of Error Instances
Error instances are ordinary
objects that inherit properties from the Error prototype object and have an
[[ErrorData]] internal slot whose value is undefined. The only
specified use of [[ErrorData]] is to identify Error, AggregateError, and
NativeError instances as Error objects within Object.prototype.toString and
Error.isError.
20.5.5 Native Error Types Used in This Standard
A new instance of one of the NativeError objects below or of the AggregateError object is thrown
when a runtime error is detected. All NativeError objects share the same structure, as described
in 20.5.6.
Indicates that one of the global URI handling functions was used in a way that is incompatible with its
definition.
20.5.6NativeError Object Structure
Each of these objects has the structure described below, differing only in the name used as the constructor name and in the
"name" property of the prototype object.
For each error object, references to NativeError in the definition should be replaced with the
appropriate error object name from 20.5.5.
creates and initializes a new NativeError object when called as a function rather than as a
constructor. A call
of the object as a function is equivalent to calling it as a constructor with the same arguments. Thus the
function call NativeError(…) is equivalent to the object creation expression
new NativeError(…) with the same arguments.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to
inherit the specified NativeError behaviour must include a super call to the
NativeErrorconstructor to create and initialize subclass instances with an
[[ErrorData]] internal slot.
20.5.6.1.1NativeError ( message [ ,
options ] )
Each NativeError function performs the following steps when called:
If NewTarget is undefined, let newTarget be the active function
object; else let newTarget be NewTarget.
The actual value of the string passed in step 2 is either
"%EvalError.prototype%", "%RangeError.prototype%",
"%ReferenceError.prototype%", "%SyntaxError.prototype%",
"%TypeError.prototype%", or "%URIError.prototype%" corresponding
to which NativeErrorconstructor is being defined.
20.5.6.2 Properties of the NativeError Constructors
The initial value of the "constructor" property of the prototype for a given
NativeErrorconstructor is the constructor itself.
20.5.6.3.2NativeError.prototype.message
The initial value of the "message" property of the prototype for a given
NativeErrorconstructor is the empty String.
20.5.6.3.3NativeError.prototype.name
The initial value of the "name" property of the prototype for a given
NativeErrorconstructor is the String value consisting of the name of the
constructor (the name
used instead of NativeError).
20.5.6.4 Properties of NativeError Instances
NativeError instances are ordinary objects that inherit properties from their
NativeError prototype object and have an [[ErrorData]] internal slot
whose value is undefined. The only specified use of [[ErrorData]] is by Object.prototype.toString (20.1.3.6) and Error.isError
(20.5.2.1)
to identify Error, AggregateError, or NativeError instances.
is the initial value of the "AggregateError" property of the global object.
creates and initializes a new AggregateError object when called as a function rather than as a
constructor. Thus the
function call AggregateError(…) is equivalent to the object creation expression
new AggregateError(…) with the same arguments.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to
inherit the specified AggregateError behaviour must include a super call to the
AggregateError constructor to create and initialize subclass instances with an
[[ErrorData]] internal slot.
The initial value of AggregateError.prototype.constructor is %AggregateError%.
20.5.7.3.2 AggregateError.prototype.message
The initial value of AggregateError.prototype.message is the empty String.
20.5.7.3.3 AggregateError.prototype.name
The initial value of AggregateError.prototype.name is "AggregateError".
20.5.7.4 Properties of AggregateError Instances
AggregateError instances are ordinary objects that inherit properties from their AggregateError prototype
object and have an [[ErrorData]] internal slot whose value is
undefined. The only specified use of [[ErrorData]] is by
Object.prototype.toString (20.1.3.6) and Error.isError
(20.5.2.1)
to identify Error, AggregateError, or NativeError instances.
20.5.8 Abstract Operations for Error Objects
20.5.8.1 InstallErrorCause ( O, options )
The abstract operation InstallErrorCause takes arguments O (an Object) and options
(an ECMAScript language value) and returns either a
normal completion containingunused or a throw completion. It
is used to create a "cause" property on O when a "cause"
property is present on options. It performs the following steps when called:
is the initial value of the "Number" property of the global object.
creates and initializes a new Number object when called as a constructor.
performs a type conversion when called as a function rather than as a constructor.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to
inherit the specified Number behaviour must include a super call to the Number constructor to create and
initialize the subclass instance with a [[NumberData]] internal slot.
21.1.1.1 Number ( value )
This function performs the following steps when called:
The value of Number.EPSILON is the Number value for the magnitude of the difference between 1 and
the smallest value greater than 1 that is representable as a Number value, which is approximately
2.2204460492503130808472633361816 × 10-16.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
21.1.2.2 Number.isFinite ( number )
This function performs the following steps when called:
This function differs from the global isNaN function (19.2.3) in that it does not convert its
argument to a Number before determining whether it is NaN.
Due to rounding behaviour necessitated by precision limitations of IEEE 754-2019, the Number value for
every integer greater than
Number.MAX_SAFE_INTEGER is shared with at least one other integer. Such large-magnitude integers are therefore not
safe, and are not
guaranteed to be exactly representable as Number values or even to be distinguishable from each other.
For example, both 9007199254740992 and 9007199254740993 evaluate to the
Number value 9007199254740992𝔽.
The value of Number.MAX_SAFE_INTEGER is 9007199254740991𝔽
(𝔽(253 - 1)).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
21.1.2.7 Number.MAX_VALUE
The value of Number.MAX_VALUE is the largest positive finite value of the Number type, which is approximately
1.7976931348623157 × 10308.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
21.1.2.8 Number.MIN_SAFE_INTEGER
Note
Due to rounding behaviour necessitated by precision limitations of IEEE 754-2019, the Number value for
every integer less than
Number.MIN_SAFE_INTEGER is shared with at least one other integer. Such large-magnitude integers are therefore not
safe, and are not
guaranteed to be exactly representable as Number values or even to be distinguishable from each other.
For example, both -9007199254740992 and -9007199254740993 evaluate to the
Number value -9007199254740992𝔽.
The value of Number.MIN_SAFE_INTEGER is -9007199254740991𝔽
(𝔽(-(253 - 1))).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
21.1.2.9 Number.MIN_VALUE
The value of Number.MIN_VALUE is the smallest positive value of the Number type, which is approximately
5 × 10-324.
In the IEEE
754-2019 double precision binary representation, the smallest possible value is a
denormalized number. If an implementation does not support denormalized values, the value of
Number.MIN_VALUE must be the smallest non-zero positive value that can actually be
represented by the implementation.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
21.1.2.10 Number.NaN
The value of Number.NaN is NaN.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
21.1.2.11 Number.NEGATIVE_INFINITY
The value of Number.NEGATIVE_INFINITY is -∞𝔽.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
21.1.2.12 Number.parseFloat ( string )
The initial value of the "parseFloat" property is %parseFloat%.
21.1.2.13 Number.parseInt ( string, radix )
The initial value of the "parseInt" property is %parseInt%.
21.1.2.14 Number.POSITIVE_INFINITY
The value of Number.POSITIVE_INFINITY is +∞𝔽.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
Unless explicitly stated otherwise, the methods of the Number prototype object defined below are not
generic and the this value passed to them must be either a Number value or an object that
has a [[NumberData]] internal slot that has been initialized to a Number value.
The phrase “this Number value” within the specification of a method refers to the result returned by
calling the abstract operation ThisNumberValue with the this value of the
method invocation passed as the argument.
21.1.3.1 Number.prototype.constructor
The initial value of Number.prototype.constructor is %Number%.
This method returns a String containing this Number value represented in decimal exponential notation
with one digit before the significand's decimal point and fractionDigits digits after the
significand's decimal point. If fractionDigits is undefined, it includes as
many significand digits as necessary to uniquely specify the Number (just like in ToString except that in this case
the Number is always output in exponential notation).
Let m be the String value consisting of f + 1 occurrences of the code unit
0x0030 (DIGIT ZERO).
Let e be 0.
Else,
If fractionDigits is not undefined, then
Let e and n be integers such that 10f ≤ n
< 10f + 1 and for which n × 10e -
f - x is as close to zero as possible. If there are two such
sets of e and n, pick the e and n for which
n × 10e - f is larger.
Else,
Let e, n, and
ff be integers such that ff ≥ 0,
10ff ≤ n < 10ff + 1, 𝔽(n × 10e
- ff) is 𝔽(x), and ff is as small as possible.
Note that the decimal representation of n has ff + 1 digits,
n is not divisible by 10, and the least significant digit of n is not
necessarily uniquely determined by these criteria.
Set f to ff.
Let m be the String value consisting of the digits of the decimal representation of
n (in order, with no leading zeroes).
For implementations that provide more accurate conversions than required by the rules above, it is
recommended that the following alternative version of step 10.b.i be used as a
guideline:
Let e, n, and f be integers such that f ≥ 0, 10f
≤ n < 10f + 1, 𝔽(n × 10e - f) is
𝔽(x), and f
is as small as possible. If there are multiple possibilities for n, choose the value of
n for which 𝔽(n × 10e - f) is
closest in value to 𝔽(x). If there are two such possible values of
n, choose the one that is even.
This method returns a String containing this Number value represented in decimal fixed-point notation
with fractionDigits digits after the decimal point. If fractionDigits is
undefined, 0 is assumed.
Let n be an integer for which n / 10f -
x is as close to zero as possible. If there are two such n, pick the larger
n.
If n = 0, let m be "0"; otherwise let m be
the String value consisting of the digits of the decimal representation of n (in order,
with no leading zeroes).
If f ≠ 0, then
Let k be the length of m.
If k ≤ f, then
Let z be the String value consisting of f + 1 - k
occurrences of the code unit 0x0030 (DIGIT ZERO).
The output of toFixed may be more precise than toString for some values
because toString only prints enough significant digits to distinguish the number from adjacent Number
values. For example,
(1000000000000000128).toString() returns "1000000000000000100",
while (1000000000000000128).toFixed(0) returns "1000000000000000128".
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement this
method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the
ECMA-402 API the following specification of this method is used:
This method produces a String value that represents this Number value formatted according to the
conventions of the host
environment's current locale. This method is implementation-defined, and it is
permissible, but not encouraged, for it to return the same thing as toString.
The meanings of the optional parameters to this method are defined in the ECMA-402 specification;
implementations that do not include ECMA-402 support must not use those parameter positions for anything
else.
This method returns a String containing this Number value represented either in decimal exponential
notation with one digit before the significand's decimal point and precision - 1 digits after the significand's decimal point or in
decimal fixed notation with precision significant digits. If precision is
undefined, it calls ToString instead.
Let m be the String value consisting of p occurrences of the code unit
0x0030 (DIGIT ZERO).
Let e be 0.
Else,
Let e and n be integers such that 10p - 1 ≤ n
< 10p and for which n × 10e - p +
1 - x is as close to zero as possible. If there are two such sets of
e and n, pick the e and n for which n ×
10e - p + 1 is larger.
Let m be the String value consisting of the digits of the decimal representation of
n (in order, with no leading zeroes).
Set m to the string-concatenation of the first e + 1
code units of m, the code unit 0x002E (FULL STOP), and the remaining p -
(e + 1) code units of m.
Else,
Set m to the string-concatenation of the code unit 0x0030
(DIGIT ZERO), the code unit 0x002E (FULL STOP), -(e + 1) occurrences of the code unit
0x0030 (DIGIT ZERO), and the String m.
The optional radix should be an integral Number value in the inclusive
interval from 2𝔽 to
36𝔽. If radix is undefined then
10𝔽 is used as the value of radix.
This method performs the following steps when called:
This method is not generic; it throws a TypeError exception if its
this value is not a Number or a
Number object. Therefore, it cannot be transferred to other kinds of objects for use as a method.
Number instances are ordinary
objects that inherit properties from the Number prototype object. Number
instances also have a [[NumberData]] internal slot. The [[NumberData]] internal slot is the Number value represented by this Number object.
is the initial value of the "BigInt" property of the global object.
performs a type conversion when called as a function rather than as a constructor.
is not intended to be used with the new operator or to be subclassed. It may be used as the
value of an extends clause of a class definition but a super call to the BigInt
constructor will cause
an exception.
21.2.1.1 BigInt ( value )
This function performs the following steps when called:
If NewTarget is not undefined, throw a TypeError exception.
The abstract operation NumberToBigInt takes argument number (a Number) and returns either a
normal completion containing a
BigInt or a throw completion. It performs the
following steps when called:
If number is not an integral Number, throw a RangeError
exception.
The phrase “this BigInt value” within the specification of a method refers to the result returned by
calling the abstract operation ThisBigIntValue with the this value of the
method invocation passed as the argument.
21.2.3.1 BigInt.prototype.constructor
The initial value of BigInt.prototype.constructor is %BigInt%.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement this
method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the
ECMA-402 API the following specification of this method is used:
This method produces a String value that represents this BigInt value formatted according to the
conventions of the host
environment's current locale. This method is implementation-defined, and it is
permissible, but not encouraged, for it to return the same thing as toString.
The meanings of the optional parameters to this method are defined in the ECMA-402 specification;
implementations that do not include ECMA-402 support must not use those parameter positions for anything
else.
21.2.3.3 BigInt.prototype.toString ( [ radix ] )
Note
The optional radix should be an integral Number value in the inclusive
interval from 2𝔽 to
36𝔽. If radix is undefined then
10𝔽 is used as the value of radix.
This method performs the following steps when called:
This method is not generic; it throws a TypeError exception if its
this value is not a BigInt or a
BigInt object. Therefore, it cannot be transferred to other kinds of objects for use as a method.
The initial value of the %Symbol.toStringTag% property is the String value
"BigInt".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true }.
21.2.4 Properties of BigInt Instances
BigInt instances are ordinary
objects that inherit properties from the BigInt prototype object. BigInt
instances also have a [[BigIntData]] internal slot. The [[BigIntData]] internal slot is the BigInt value represented by this BigInt object.
21.3 The Math Object
The Math object:
is %Math%.
is the initial value of the "Math" property of the global object.
does not have a [[Construct]] internal method; it cannot be used as a constructor with the
new operator.
does not have a [[Call]] internal method; it cannot be invoked as a function.
Note
In this specification, the phrase “the Number value forx” has a technical meaning defined
in 6.1.6.1.
21.3.1 Value Properties of the Math Object
21.3.1.1 Math.E
The Number value
fore, the base of the natural logarithms, which is approximately
2.7182818284590452354.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
21.3.1.2 Math.LN10
The Number value
for the natural logarithm of 10, which is approximately 2.302585092994046.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
21.3.1.3 Math.LN2
The Number value
for the natural logarithm of 2, which is approximately 0.6931471805599453.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
21.3.1.4 Math.LOG10E
The Number value
for the base-10 logarithm of e, the base of the natural logarithms; this value
is approximately 0.4342944819032518.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
Note
The value of Math.LOG10E is approximately the reciprocal of the value of
Math.LN10.
21.3.1.5 Math.LOG2E
The Number value
for the base-2 logarithm of e, the base of the natural logarithms; this value is
approximately 1.4426950408889634.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
Note
The value of Math.LOG2E is approximately the reciprocal of the value of
Math.LN2.
21.3.1.6 Math.PI
The Number value
for π, the ratio of the circumference of a circle to its diameter, which is
approximately 3.1415926535897932.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
21.3.1.7 Math.SQRT1_2
The Number value
for the square root of ½, which is approximately 0.7071067811865476.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
Note
The value of Math.SQRT1_2 is approximately the reciprocal of the value of
Math.SQRT2.
21.3.1.8 Math.SQRT2
The Number value
for the square root of 2, which is approximately 1.4142135623730951.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
21.3.1.9 Math [ %Symbol.toStringTag% ]
The initial value of the %Symbol.toStringTag% property is the String value
"Math".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true }.
21.3.2 Function Properties of the Math Object
Note
The behaviour of the functions acos, acosh, asin,
asinh, atan, atanh, atan2, cbrt,
cos, cosh, exp, expm1, hypot,
log, log1p, log2, log10, pow,
random, sin, sinh, tan, and tanh is not
precisely specified here except to require specific results for certain argument values that represent
boundary cases of interest. For other argument values, these functions are intended to compute
approximations to the results of familiar mathematical functions, but some latitude is allowed in the
choice of approximation algorithms. The general intent is that an implementer should be able to use the
same mathematical library for ECMAScript on a given hardware platform that is available to C programmers
on that platform.
Although the choice of algorithms is left to the implementation, it is recommended (but not specified
by this standard) that implementations use the approximation algorithms for IEEE 754-2019
arithmetic contained in fdlibm, the freely distributable mathematical library from Sun
Microsystems (http://www.netlib.org/fdlibm).
21.3.2.1 Math.abs ( x )
This function returns the absolute value of x; the result has the same magnitude as
x but has positive sign.
This function returns the inverse tangent of the quotient y /
x of the arguments y and x, where the signs of y
and x are used to determine the quadrant of the result. Note that it is intentional and
traditional for the two-argument inverse tangent function that the argument named y be first
and the argument named x be second. The result is expressed in radians and is in the inclusive
interval from -π to +π.
This function returns the result of subtracting 1 from the exponential function of x
(e raised to the power of x, where e is the base of the natural
logarithms). The result is computed in a way that is accurate even when the value of x is close
to 0.
This function returns the greatest (closest to +∞) integral Number value that is not greater than x. If
x is already an integral Number, the result is x.
Let n16 be the result of converting n to IEEE 754-2019 binary16 format using
roundTiesToEven mode.
Let n64 be the result of converting n16 to IEEE 754-2019 binary64 format.
Return the ECMAScript Number value corresponding to n64.
Note
This operation is not the same as casting to binary32 and then to binary16 because of the possibility
of double-rounding: consider the number k =
1.00048828125000022204𝔽, for example, for which
Math.f16round(k) is 1.0009765625𝔽, but
Math.f16round(Math.fround(k)) is 1𝔽.
Not all platforms provide native support for casting from binary64 to binary16. There are various
libraries which can provide this, including the MIT-licensed half library. Alternatively, it is possible to first cast
from binary64 to binary32 under roundTiesToEven and then check whether the result could lead to
incorrect double-rounding. The cases which could can be handled explicitly by adjusting the mantissa
of the binary32 value so that it is the value which would be produced by performing the initial cast
under roundTiesToOdd. Casting the adjusted value to binary16 under roundTiesToEven then produces the
correct value.
21.3.2.19 Math.hypot ( ...args )
Given zero or more arguments, this function returns the square root of the sum of squares of its
arguments.
Implementations should take care to avoid the loss of precision from overflows and underflows that
are prone to occur in naive implementations when this function is called with two or more arguments.
21.3.2.20 Math.imul ( x, y )
This function performs the following steps when called:
This function returns a Number value with positive sign, greater than or equal to
+0𝔽 but strictly less than 1𝔽, chosen
randomly or pseudo randomly with approximately uniform distribution over that range, using an implementation-defined algorithm or strategy.
Each Math.random function created for distinct realms must produce a distinct sequence of values from successive calls.
21.3.2.29 Math.round ( x )
This function returns the Number value that is closest to x and is integral. If two integral Numbers are
equally close to x, then the result is the Number value that is closer to +∞. If x
is already integral, the result is x.
Return the integral
Number closest to n, preferring the Number closer to +∞ in the case of a
tie.
Note 1
Math.round(3.5) returns 4, but Math.round(-3.5) returns -3.
Note 2
The value of Math.round(x) is not always the same as the value of
Math.floor(x + 0.5). When x is -0𝔽 or
x is less than -0𝔽 but greater than or equal to
-0.5𝔽, Math.round(x) returns
-0𝔽, but Math.floor(x + 0.5) returns
+0𝔽. Math.round(x) may also differ from the value of
Math.floor(x + 0.5)because of internal rounding when computing x + 0.5.
21.3.2.30 Math.sign ( x )
This function returns the sign of x, indicating whether x is positive, negative, or
zero.
If n is not finite or n is either +0𝔽
or -0𝔽, return n.
If n < 1𝔽 and n >
+0𝔽, return +0𝔽.
If n < -0𝔽 and n >
-1𝔽, return -0𝔽.
Return the integral
Number nearest n in the direction of +0𝔽.
21.4 Date Objects
21.4.1 Overview of Date Objects and Definitions of Abstract Operations
The following abstract operations operate on
time values (defined in 21.4.1.1). Note that, in every case, if any
argument to one of these functions is NaN, the result will be NaN.
21.4.1.1 Time Values and Time Range
Time measurement in ECMAScript is analogous to time measurement in POSIX, in particular sharing
definition in terms of the proleptic Gregorian calendar, an epoch of
midnight at the beginning of 1 January 1970 UTC, and an accounting of every day as comprising exactly
86,400 seconds (each of which is 1000 milliseconds long).
An ECMAScript time value is a Number, either a finiteintegral Number representing an instant in
time to millisecond precision or NaN representing no specific instant. A time value
that is a multiple of 24 × 60 × 60 × 1000 = 86,400,000 (i.e., is
86,400,000 × d for some integerd) represents the instant at the start of the UTC
day that follows the epoch by
d whole UTC days (preceding the epoch for negative d). Every other finite time value t is defined relative to
the greatest preceding time value s that is such a multiple, and represents the instant that
occurs within the same UTC day as s but follows it by (t - s)
milliseconds.
Time values do not account for UTC leap seconds—there are no time values representing instants within
positive leap seconds, and there are time values representing instants removed from the UTC timeline by
negative leap seconds. However, the definition of time values nonetheless yields piecewise alignment with
UTC, with discontinuities only at leap second boundaries and zero difference outside of leap seconds.
A Number can exactly represent all integers from -9,007,199,254,740,992 to 9,007,199,254,740,992
(21.1.2.8 and 21.1.2.6). A time value supports a slightly smaller
range of -8,640,000,000,000,000 to 8,640,000,000,000,000 milliseconds. This yields a supported time value
range of exactly -100,000,000 days to 100,000,000 days relative to midnight at the beginning of 1 January
1970 UTC.
The exact moment of midnight at the beginning of 1 January 1970 UTC is represented by the time value
+0𝔽.
Note
In the proleptic Gregorian calendar, leap years are precisely those which are both divisible by 4 and
either divisible by 400 or not divisible by 100.
The 400 year cycle of the proleptic Gregorian calendar contains 97 leap years. This yields an average
of 365.2425 days per year, which is 31,556,952,000 milliseconds. Therefore, the maximum range a Number
could represent exactly with millisecond precision is approximately -285,426 to 285,426 years relative
to 1970. The smaller range supported by a time value as specified in this section is approximately
-273,790 to 273,790 years relative to 1970.
21.4.1.2 Time-related Constants
These constants are referenced by algorithms in the following sections.
The abstract operation Day takes argument t (a finitetime value) and returns an
integral
Number. It returns the day number of the day in which t falls. It performs
the following steps when called:
The abstract operation TimeWithinDay takes argument t (a finitetime value) and returns an
integral
Number in the interval from +0𝔽 (inclusive) to
msPerDay (exclusive). It
returns the number of milliseconds since the start of the day in which t falls. It performs the
following steps when called:
The abstract operation DaysInYear takes argument y (an integral Number) and returns
365𝔽 or 366𝔽. It returns the number of days
in year y. Leap years have 366 days; all other years have 365. It performs the following steps
when called:
The abstract operation DayFromYear takes argument y (an integral Number) and returns an integral Number. It
returns the day number of the first day of year y. It performs the following steps when called:
NOTE: In the following steps,
numYears1, numYears4, numYears100, and numYears400
represent the number of years divisible by 1, 4, 100, and 400, respectively, that occur between the
epoch and the start of year
y. The number is negative if y is before the epoch.
The abstract operation TimeFromYear takes argument y (an integral Number) and returns a time
value. It returns the time value of the start of year
y. It performs the following steps when called:
The abstract operation YearFromTime takes argument t (a finitetime value) and returns an
integral
Number. It returns the year in which t falls. It performs the following
steps when called:
The abstract operation InLeapYear takes argument t (a finitetime value) and returns
+0𝔽 or 1𝔽. It returns
1𝔽 if t is within a leap year and
+0𝔽 otherwise. It performs the following steps when called:
The abstract operation MonthFromTime takes argument t (a finitetime value) and returns an
integral
Number in the inclusive interval from +0𝔽
to 11𝔽. It returns a Number identifying the month in which t
falls. A month value of +0𝔽 specifies January;
1𝔽 specifies February; 2𝔽 specifies March;
3𝔽 specifies April; 4𝔽 specifies May;
5𝔽 specifies June; 6𝔽 specifies July;
7𝔽 specifies August; 8𝔽 specifies September;
9𝔽 specifies October; 10𝔽 specifies
November; and 11𝔽 specifies December. Note that MonthFromTime(+0𝔽) =
+0𝔽, corresponding to Thursday, 1 January 1970. It performs the
following steps when called:
The abstract operation DateFromTime takes argument t (a finitetime value) and returns an
integral
Number in the inclusive interval from 1𝔽 to
31𝔽. It returns the day of the month in which t falls. It
performs the following steps when called:
The abstract operation WeekDay takes argument t (a finitetime value) and returns an
integral
Number in the inclusive interval from +0𝔽
to 6𝔽. It returns a Number identifying the day of the week in which
t falls. A weekday value of +0𝔽 specifies Sunday;
1𝔽 specifies Monday; 2𝔽 specifies Tuesday;
3𝔽 specifies Wednesday; 4𝔽 specifies
Thursday; 5𝔽 specifies Friday; and 6𝔽
specifies Saturday. Note that WeekDay(+0𝔽) =
4𝔽, corresponding to Thursday, 1 January 1970. It performs the
following steps when called:
The abstract operation HourFromTime takes argument t (a finitetime value) and returns an
integral
Number in the inclusive interval from +0𝔽
to 23𝔽. It returns the hour of the day in which t falls. It
performs the following steps when called:
The abstract operation MinFromTime takes argument t (a finitetime value) and returns an
integral
Number in the inclusive interval from +0𝔽
to 59𝔽. It returns the minute of the hour in which t falls. It
performs the following steps when called:
The abstract operation SecFromTime takes argument t (a finitetime value) and returns an
integral
Number in the inclusive interval from +0𝔽
to 59𝔽. It returns the second of the minute in which t falls. It
performs the following steps when called:
The abstract operation msFromTime takes argument t (a finitetime value) and returns an
integral
Number in the inclusive interval from +0𝔽
to 999𝔽. It returns the millisecond of the second in which t
falls. It performs the following steps when called:
Time zones in ECMAScript are represented by time zone
identifiers, which are Strings composed entirely of code units in the inclusive
interval from 0x0000 to 0x007F.
Time zones supported by an ECMAScript implementation may be available named time zones, represented by the [[Identifier]] field of the Time Zone Identifier Records
returned by AvailableNamedTimeZoneIdentifiers, or
offset time zones, represented by Strings for which
IsTimeZoneOffsetString returns
true.
A primary time zone identifier is the
preferred identifier for an available named time zone.
A non-primary time zone identifier
is an identifier for an available named time zone that is not a primary time zone identifier.
An available named time zone
identifier is either a primary time zone identifier or a non-primary time zone identifier.
Each available named time zone identifier is associated with exactly one available named time zone.
Each available named time zone is associated with exactly one primary time zone identifier and zero or
more non-primary time zone identifiers.
ECMAScript implementations must support an available named time zone with the identifier
"UTC", which must be the primary time zone identifier for the UTC time zone.
In addition, implementations may support any number of other available named time zones.
Implementations that follow the requirements for time zones as described in the ECMA-402
Internationalization API specification are called time zone aware.
Time zone aware implementations must support available named time zones corresponding to the Zone and Link
names of the IANA Time Zone Database, and only such names.
In time zone aware implementations, a primary time zone identifier is a Zone name, and a non-primary time
zone identifier is a Link name, respectively, in the IANA Time Zone Database except as specifically
overridden by AvailableNamedTimeZoneIdentifiers as
specified in the ECMA-402 specification.
Implementations that do not support the entire IANA Time Zone Database are still recommended to use IANA
Time Zone Database names as identifiers to represent time zones.
When the input represents a local time occurring more than once because of a negative time zone transition
(e.g. when daylight saving time ends or the time zone offset is decreased due to a time zone rule change),
the returned List will have more than one element and
will be sorted by ascending numerical value.
When the input represents a local time skipped because of a positive time zone transition (e.g. when
daylight saving time begins or the time zone offset is increased due to a time zone rule change), the
returned List will be empty.
Otherwise, the returned List will have one element.
The default implementation of GetNamedTimeZoneEpochNanoseconds, to be used for ECMAScript implementations
that do not include local political rules for any time zones, performs the following steps when called:
1:30 AM on 5 November 2017 in America/New_York is repeated twice, so
GetNamedTimeZoneEpochNanoseconds("America/New_York", 2017, 11, 5, 1, 30, 0, 0, 0,
0) would return a List of length 2 in which the first
element represents 05:30 UTC (corresponding with 01:30 US Eastern Daylight Time at UTC offset -04:00)
and the second element represents 06:30 UTC (corresponding with 01:30 US Eastern Standard Time at UTC
offset -05:00).
2:30 AM on 12 March 2017 in America/New_York does not exist, so
GetNamedTimeZoneEpochNanoseconds("America/New_York", 2017, 3, 12, 2, 30, 0, 0, 0,
0) would return an empty List.
The implementation-defined abstract operation
GetNamedTimeZoneOffsetNanoseconds takes arguments timeZoneIdentifier (a String) and
epochNanoseconds (a BigInt) and returns an integer.
The returned integer
represents the offset from UTC of the named time zone identified by timeZoneIdentifier, at the
instant corresponding with epochNanoseconds relative to the epoch, both in nanoseconds.
The default implementation of GetNamedTimeZoneOffsetNanoseconds, to be used for ECMAScript
implementations that do not include local political rules for any time zones, performs the following steps
when called:
Time zone
aware implementations, including all implementations that implement the ECMA-402
Internationalization API, must implement the AvailableNamedTimeZoneIdentifiers abstract operation as
specified in the ECMA-402 specification.
For implementations that are not time zone aware, AvailableNamedTimeZoneIdentifiers
performs the following steps when called:
If the implementation does not include local political rules for any time zones, then
The implementation-defined abstract operation
SystemTimeZoneIdentifier takes no arguments and returns a String.
It returns a String representing the host environment's current time zone, which is either a String
representing a UTC offset for which IsTimeZoneOffsetString returns
true, or a primary time zone identifier.
It performs the following steps when called:
If the implementation only supports the UTC time zone, return "UTC".
To ensure the level of functionality that implementations commonly provide in the methods of the Date
object, it is recommended that SystemTimeZoneIdentifier return an IANA time zone name corresponding to
the host
environment's time zone setting, if such a thing exists.
GetNamedTimeZoneEpochNanoseconds and
GetNamedTimeZoneOffsetNanoseconds
must reflect the local political rules for standard time and daylight saving time in that time zone,
if such rules exist.
For example, if the host environment is a browser on a system where the user
has chosen US Eastern Time as their time zone, SystemTimeZoneIdentifier returns
"America/New_York".
21.4.1.25 LocalTime ( t )
The abstract operation LocalTime takes argument t (a finitetime value) and returns an
integral
Number.
It converts t from UTC to local time.
The local political rules for standard time and daylight saving time in effect at t should be
used to determine the result in the way specified in this section.
It performs the following steps when called:
Two different input time valuestUTC are converted to the same local time tlocal at a negative time zone transition when there are
repeated times (e.g. the daylight saving time ends or the time zone adjustment is decreased.).
LocalTime(UTC(tlocal)) is not necessarily
always equal to tlocal. Correspondingly,
UTC(LocalTime(tUTC)) is not
necessarily always equal to tUTC.
21.4.1.26 UTC ( t )
The abstract operation UTC takes argument t (a Number) and returns a time
value.
It converts t from local time to a UTC time value.
The local political rules for standard time and daylight saving time in effect at t should be
used to determine the result in the way specified in this section.
It performs the following steps when called:
NOTE: The following steps ensure that when t represents local time repeating multiple
times at a negative time zone transition (e.g. when the daylight saving time ends or the time zone
offset is decreased due to a time zone rule change) or skipped local time at a positive time zone
transition (e.g. when the daylight saving time starts or the time zone offset is increased due to
a time zone rule change), t is interpreted using the time zone offset before the
transition.
If possibleInstants is not empty, then
Let disambiguatedInstant be possibleInstants[0].
Else,
NOTE: t represents a local time skipped at a positive time zone transition (e.g.
due to daylight saving time starting or a time zone rule change increasing the UTC offset).
Input t is nominally a time value but may be any Number value.
The algorithm must not limit t to the time value range, so that
inputs corresponding with a boundary of the time value range can be
supported regardless of local UTC offset.
For example, the maximum time value is 8.64 × 1015,
corresponding with "+275760-09-13T00:00:00Z".
In an environment where the local time zone offset is ahead of UTC by 1 hour at that instant, it is
represented by the larger input of 8.64 × 1015 + 3.6 × 106, corresponding with
"+275760-09-13T01:00:00+01:00".
1:30 AM on 5 November 2017 in America/New_York is repeated twice (fall backward), but it must be
interpreted as 1:30 AM UTC-04 instead of 1:30 AM UTC-05.
In UTC(TimeClip(MakeDate(MakeDay(2017, 10, 5), MakeTime(1, 30, 0, 0)))), the value of
offsetMs is -4 × msPerHour.
2:30 AM on 12 March 2017 in America/New_York does not exist, but it must be interpreted as 2:30 AM
UTC-05 (equivalent to 3:30 AM UTC-04).
In UTC(TimeClip(MakeDate(MakeDay(2017, 2, 12), MakeTime(2, 30, 0, 0)))), the value of
offsetMs is -5 × msPerHour.
Note 2
UTC(LocalTime(tUTC)) is not
necessarily always equal to tUTC.
Correspondingly, LocalTime(UTC(tlocal)) is
not necessarily always equal to tlocal.
21.4.1.27 MakeTime ( hour, min, sec,
ms )
The abstract operation MakeTime takes arguments hour (a Number), min (a Number),
sec (a Number), and ms (a Number) and returns a Number. It calculates a number of
milliseconds. It performs the following steps when called:
The arithmetic in MakeTime is floating-point arithmetic, which is not associative, so the operations
must be performed in the correct order.
21.4.1.28 MakeDay ( year, month, date )
The abstract operation MakeDay takes arguments year (a Number), month (a Number),
and date (a Number) and returns a Number. It calculates a number of days. It performs the
following steps when called:
If year is not finite, month is not finite, or date is not finite, return
NaN.
The abstract operation MakeDate takes arguments day (a Number) and time (a Number)
and returns a Number. It calculates a number of milliseconds. It performs the following steps when called:
If day is not finite or time is not finite, return NaN.
The abstract operation MakeFullYear takes argument year (a Number) and returns an integral Number or
NaN. It returns the full year associated with the integer part of year, interpreting any
value in the inclusive
interval from 0 to 99 as a count of years since the start of 1900. For alignment with
the proleptic Gregorian calendar, "full year" is defined as the signed count of complete years since the
start of year 0 (1 B.C.). It performs the following steps when called:
The abstract operation TimeClip takes argument time (a Number) and returns a Number. It
calculates a number of milliseconds. It performs the following steps when called:
ECMAScript defines a string interchange format for date-times based upon a simplification of the ISO 8601
calendar date extended format. The format is as follows: YYYY-MM-DDTHH:mm:ss.sssZ
Where the elements are as follows:
YYYY
is the year in the proleptic Gregorian calendar as four decimal digits from 0000 to 9999, or as an
expanded
year of "+" or "-" followed by six
decimal digits.
-
"-" (hyphen) appears literally twice in the string.
MM
is the month of the year as two decimal digits from 01 (January) to 12 (December).
DD
is the day of the month as two decimal digits from 01 to 31.
T
"T" appears literally in the string, to indicate the beginning of the time
element.
HH
is the number of complete hours that have passed since midnight as two decimal digits from 00 to
24.
:
":" (colon) appears literally twice in the string.
mm
is the number of complete minutes since the start of the hour as two decimal digits from 00 to 59.
ss
is the number of complete seconds since the start of the minute as two decimal digits from 00 to
59.
.
"." (dot) appears literally in the string.
sss
is the number of complete milliseconds since the start of the second as three decimal digits.
Z
is the UTC offset representation specified as "Z" (for UTC with no offset) or
as either "+" or "-" followed by a time expression
HH:mm (a subset of the time zone offset string
format for indicating local time ahead of or behind UTC, respectively)
This format includes date-only forms:
YYYY
YYYY-MM
YYYY-MM-DD
It also includes “date-time” forms that consist of one of the above date-only forms immediately followed
by one of the following time forms with an optional UTC offset representation appended:
THH:mm
THH:mm:ss
THH:mm:ss.sss
A string containing out-of-bounds or nonconforming elements is not a valid instance of this format.
Note 1
As every day both starts and ends with midnight, the two notations 00:00 and
24:00 are available to distinguish the two midnights that can be associated with one
date. This means that the following two notations refer to exactly the same point in time:
1995-02-04T24:00 and 1995-02-05T00:00. This interpretation of the latter
form as "end of a calendar day" is consistent with ISO 8601, even though that specification reserves
it for describing time intervals and does not permit it within representations of single points in
time.
Note 2
There exists no international standard that specifies abbreviations for civil time zones like CET,
EST, etc. and sometimes the same abbreviation is even used for two very different time zones. For this
reason, both ISO 8601 and this format specify numeric representations of time zone offsets.
21.4.1.32.1 Expanded Years
Covering the full time value range of approximately 273,790
years forward or backward from 1 January 1970 (21.4.1.1) requires representing
years before 0 or after 9999. ISO 8601 permits expansion of the year representation, but only by mutual
agreement of the partners in information interchange. In the simplified ECMAScript format, such an
expanded year representation shall have 6 digits and is always prefixed with a + or - sign. The year 0
is considered positive and must be prefixed with a + sign. The representation of the year 0 as -000000
is invalid. Strings matching the Date Time String Format with expanded years
representing instants in time outside the range of a time value are treated as
unrecognizable by Date.parse and cause that function to return
NaN without falling back to implementation-specific behaviour or heuristics.
Note
Examples of date-time values with expanded years:
-271821-04-20T00:00:00Z
271822 B.C.
-000001-01-01T00:00:00Z
2 B.C.
+000000-01-01T00:00:00Z
1 B.C.
+000001-01-01T00:00:00Z
1 A.D.
+001970-01-01T00:00:00Z
1970 A.D.
+002009-12-15T00:00:00Z
2009 A.D.
+275760-09-13T00:00:00Z
275760 A.D.
21.4.1.33 Time Zone Offset String Format
ECMAScript defines a string interchange format for UTC offsets, derived from ISO 8601.
The format is described by the following grammar.
The abstract operation IsTimeZoneOffsetString takes argument offsetString (a String) and
returns a Boolean. The return value indicates whether offsetString conforms to the grammar
given by UTCOffset. It performs the
following steps when called:
The abstract operation ParseTimeZoneOffsetString takes argument offsetString (a String) and
returns an integer. The
return value is the UTC offset, as a number of nanoseconds, that corresponds to the String
offsetString. It performs the following steps when called:
If parsedSign is the single code point U+002D (HYPHEN-MINUS), then
Let sign be -1.
Else,
Let sign be 1.
NOTE: Applications of StringToNumber below do not lose precision, since each
of the parsed values is guaranteed to be a sufficiently short string of decimal digits.
is the initial value of the "Date" property of the global object.
creates and initializes a new Date when called as a constructor.
returns a String representing the current time (UTC) when called as a function rather than as a
constructor.
is a function whose behaviour differs based upon the number and types of its arguments.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to
inherit the specified Date behaviour must include a super call to the Date constructor to create and
initialize the subclass instance with a [[DateValue]] internal slot.
21.4.2.1 Date ( ...values )
This function performs the following steps when called:
If NewTarget is undefined, then
Let now be the time value (UTC) identifying the current
time.
This function returns the time value designating the UTC date and time of
the occurrence of the call to it.
21.4.3.2 Date.parse ( string )
This function applies the ToString operator to its argument. If ToString results in an abrupt completion the Completion Record is immediately
returned. Otherwise, this function interprets the resulting String as a date and time; it returns a
Number, the UTC time value corresponding to the date and time.
The String may be interpreted as a local time, a UTC time, or a time in some other time zone, depending on
the contents of the String. The function first attempts to parse the String according to the format
described in Date Time String Format (21.4.1.32), including expanded years. If the String
does not conform to that format the function may fall back to any implementation-specific heuristics or
implementation-specific date formats. Strings that are unrecognizable or contain out-of-bounds format
element values shall cause this function to return NaN.
If the String conforms to the Date Time String Format, substitute values take the
place of absent format elements. When the MM or DD elements are absent,
"01" is used. When the HH, mm, or ss elements
are absent, "00" is used. When the sss element is absent,
"000" is used. When the UTC offset representation is absent, date-only forms are
interpreted as a UTC time and date-time forms are interpreted as a local time.
If x is any Date whose milliseconds amount is zero within a particular implementation of
ECMAScript, then all of the following expressions should produce the same numeric value in that
implementation, if all the properties referenced have their initial values:
is not required to produce the same Number value as the preceding three expressions and, in general, the
value produced by this function is implementation-defined when given any String value that
does not conform to the Date Time String Format (21.4.1.32) and that could not be
produced in that implementation by the toString or toUTCString method.
This function differs from the Date constructor in two ways: it returns a time value as a Number, rather than creating
a Date, and it interprets the arguments in UTC rather than as local time.
Unless explicitly defined otherwise, the methods of the Date prototype object defined below are not generic
and the this value passed to them must be an object that has a [[DateValue]] internal slot that has been initialized to a time
value.
21.4.4.1 Date.prototype.constructor
The initial value of Date.prototype.constructor is %Date%.
21.4.4.2 Date.prototype.getDate ( )
This method performs the following steps when called:
If month is not present, this method behaves as if month was present with the
value getMonth(). If date is not present, it behaves as if date was
present with the value getDate().
21.4.4.22 Date.prototype.setHours ( hour [ , min [ ,
sec [ , ms ] ] ] )
This method performs the following steps when called:
If min is not present, this method behaves as if min was present with the value
getMinutes(). If sec is not present, it behaves as if sec was
present with the value getSeconds(). If ms is not present, it behaves as if
ms was present with the value getMilliseconds().
21.4.4.23 Date.prototype.setMilliseconds ( ms )
This method performs the following steps when called:
If sec is not present, this method behaves as if sec was present with the value
getSeconds(). If ms is not present, this behaves as if ms was
present with the value getMilliseconds().
21.4.4.25 Date.prototype.setMonth ( month [ , date ] )
This method performs the following steps when called:
If month is not present, this method behaves as if month was present with the
value getUTCMonth(). If date is not present, it behaves as if date
was present with the value getUTCDate().
21.4.4.30 Date.prototype.setUTCHours ( hour [ , min [
, sec [ , ms ] ] ] )
This method performs the following steps when called:
If min is not present, this method behaves as if min was present with the value
getUTCMinutes(). If sec is not present, it behaves as if sec was
present with the value getUTCSeconds(). If ms is not present, it behaves as if
ms was present with the value getUTCMilliseconds().
21.4.4.31 Date.prototype.setUTCMilliseconds ( ms )
This method performs the following steps when called:
If sec is not present, this method behaves as if sec was present with the value
getUTCSeconds(). If ms is not present, it behaves as if ms was
present with the value return by getUTCMilliseconds().
21.4.4.33 Date.prototype.setUTCMonth ( month [ , date
] )
This method performs the following steps when called:
If tv corresponds with a year that cannot be represented in the Date Time
String Format, throw a RangeError exception.
Return a String representation of tv in the Date Time String Format on the
UTC time scale, including all format elements and the UTC offset representation
"Z".
21.4.4.37 Date.prototype.toJSON ( key )
This method provides a String representation of a Date for use by JSON.stringify (25.5.2).
This method is intentionally generic; it does not require that its this value be a
Date. Therefore, it can be transferred to other kinds of objects for use as a method. However, it does
require that any such object have a toISOString method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement this
method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the
ECMA-402 API the following specification of this method is used:
This method returns a String value. The contents of the String are implementation-defined, but are intended to represent
the “date” portion of the Date in the current time zone in a convenient, human-readable form that
corresponds to the conventions of the host environment's current locale.
The meaning of the optional parameters to this method are defined in the ECMA-402 specification;
implementations that do not include ECMA-402 support must not use those parameter positions for anything
else.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement this
method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the
ECMA-402 API the following specification of this method is used:
This method returns a String value. The contents of the String are implementation-defined, but are intended to represent
the Date in the current time zone in a convenient, human-readable form that corresponds to the conventions
of the host
environment's current locale.
The meaning of the optional parameters to this method are defined in the ECMA-402 specification;
implementations that do not include ECMA-402 support must not use those parameter positions for anything
else.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement this
method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the
ECMA-402 API the following specification of this method is used:
This method returns a String value. The contents of the String are implementation-defined, but are intended to represent
the “time” portion of the Date in the current time zone in a convenient, human-readable form that
corresponds to the conventions of the host environment's current locale.
The meaning of the optional parameters to this method are defined in the ECMA-402 specification;
implementations that do not include ECMA-402 support must not use those parameter positions for anything
else.
21.4.4.41 Date.prototype.toString ( )
This method performs the following steps when called:
For any Date d such that d.[[DateValue]] is evenly
divisible by 1000, the result of Date.parse(d.toString()) = d.valueOf(). See
21.4.3.2.
Note 2
This method is not generic; it throws a TypeError exception if its
this value is not a Date. Therefore, it cannot be transferred to other kinds of
objects for use as a method.
21.4.4.41.1 TimeString ( tv )
The abstract operation TimeString takes argument tv (a Number, but not
NaN) and returns a String. It performs the following steps when called:
Return the string-concatenation of weekday, the code
unit 0x0020 (SPACE), month, the code unit 0x0020 (SPACE), day, the code unit
0x0020 (SPACE), yearSign, and paddedYear.
Table 63: Names of days of the week
Number
Name
+0𝔽
"Sun"
1𝔽
"Mon"
2𝔽
"Tue"
3𝔽
"Wed"
4𝔽
"Thu"
5𝔽
"Fri"
6𝔽
"Sat"
Table 64: Names of months of the year
Number
Name
+0𝔽
"Jan"
1𝔽
"Feb"
2𝔽
"Mar"
3𝔽
"Apr"
4𝔽
"May"
5𝔽
"Jun"
6𝔽
"Jul"
7𝔽
"Aug"
8𝔽
"Sep"
9𝔽
"Oct"
10𝔽
"Nov"
11𝔽
"Dec"
21.4.4.41.3 TimeZoneString ( tv )
The abstract operation TimeZoneString takes argument tv (an integral Number) and
returns a String. It performs the following steps when called:
Let tzName be an implementation-defined string that is either the
empty String or the string-concatenation of the code unit 0x0020
(SPACE), the code unit 0x0028 (LEFT PARENTHESIS), an implementation-defined timezone
name, and the code unit 0x0029 (RIGHT PARENTHESIS).
Return the string-concatenation of offsetSign,
offsetHour, offsetMin, and tzName.
21.4.4.41.4 ToDateString ( tv )
The abstract operation ToDateString takes argument tv (an integral Number or NaN)
and returns a String. It performs the following steps when called:
This method returns a String value representing the instant in time corresponding to the
this value. The format of the String is based upon "HTTP-date" from RFC 7231,
generalized to support the full range of times supported by ECMAScript Dates.
Return the string-concatenation of weekday,
",", the code unit 0x0020 (SPACE), day, the code unit 0x0020 (SPACE),
month, the code unit 0x0020 (SPACE), yearSign, paddedYear, the code
unit 0x0020 (SPACE), and TimeString(tv).
21.4.4.44 Date.prototype.valueOf ( )
This method performs the following steps when called:
21.4.4.45 Date.prototype [ %Symbol.toPrimitive% ] ( hint )
This method is called by ECMAScript language operators to convert a Date to a primitive value. The
allowed values for hint are "default", "number", and
"string". Dates are unique among built-in ECMAScript object in that they treat
"default" as being equivalent to "string", All other built-in
ECMAScript objects treat "default" as being equivalent to "number".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true }.
The value of the "name" property of this method is
"[Symbol.toPrimitive]".
21.4.5 Properties of Date Instances
Date instances are ordinary
objects that inherit properties from the Date prototype object. Date instances
also have a [[DateValue]] internal slot. The [[DateValue]]
internal slot is the time value represented by this Date.
is the initial value of the "String" property of the global object.
creates and initializes a new String object when called as a constructor.
performs a type conversion when called as a function rather than as a constructor.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to
inherit the specified String behaviour must include a super call to the String constructor to create and
initialize the subclass instance with a [[StringData]] internal slot.
22.1.1.1 String ( value )
This function performs the following steps when called:
This function may be called with a variable number of arguments. The first argument is
template and the remainder of the arguments form the Listsubstitutions.
It performs the following steps when called:
Let substitutionCount be the number of elements in substitutions.
This function is intended for use as a tag function of a Tagged Template (13.3.11).
When called as such, the first argument will be a well formed template object and the rest parameter
will contain the substitution values.
Unless explicitly stated otherwise, the methods of the String prototype object defined below are not
generic and the this value passed to them must be either a String value or an object that
has a [[StringData]] internal slot that has been initialized to a String value.
This method returns a single element String containing the code unit at index pos within
the String value resulting from converting this object to a String. If there is no element at that
index, the result is the empty String. The result is a String value, not a String
object.
If pos is an integral Number, then the result of
x.charAt(pos) is equivalent to the result of x.substring(pos, pos + 1).
This method performs the following steps when called:
If position < 0 or position ≥ size, return the empty String.
Return the substring
of S from position to position + 1.
Note 2
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.3 String.prototype.charCodeAt ( pos )
Note 1
This method returns a Number (a non-negative integral Number less than 216) that is the
numeric value of the code unit at index pos within the String resulting from converting
this object to a String. If there is no element at that index, the result is NaN.
This method performs the following steps when called:
Return the Number
value for the numeric value of the code unit at index position within
the String S.
Note 2
This method is intentionally generic; it does not require that its this value be a
String object. Therefore it can be transferred to other kinds of objects for use as a method.
22.1.3.4 String.prototype.codePointAt ( pos )
Note 1
This method returns a non-negative integral Number less than or equal to
0x10FFFF𝔽 that is the numeric value of the UTF-16 encoded code point
(6.1.4) starting at the string
element at index pos within the String resulting from converting this object to a String.
If there is no element at that index, the result is undefined. If a valid UTF-16
surrogate
pair does not begin at pos, the result is the code unit at
pos.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be a
String object. Therefore it can be transferred to other kinds of objects for use as a method.
22.1.3.5 String.prototype.concat ( ...args )
Note 1
When this method is called it returns the String value consisting of the code units of the
this value (converted to a String) followed by the code units of each of the
arguments converted to a String. The result is a String value, not a String
object.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be a
String object. Therefore it can be transferred to other kinds of objects for use as a method.
22.1.3.6 String.prototype.constructor
The initial value of String.prototype.constructor is %String%.
If endPosition is undefined, let pos be len;
else let pos be ? ToIntegerOrInfinity(endPosition).
Let end be the result of clampingpos between 0 and len.
Let searchLength be the length of searchStr.
If searchLength = 0, return true.
Let start be end - searchLength.
If start < 0, return false.
Let substring be the substring of S from start to
end.
If substring is searchStr, return true.
Return false.
Note 1
This method returns true if the sequence of code units of searchString
converted to a String is the same as the corresponding code units of this object (converted to a
String) starting at endPosition - length(this). Otherwise it returns
false.
Note 2
Throwing an exception if the first argument is a RegExp is specified in order to allow future
editions to define extensions that allow such argument values.
Note 3
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.8 String.prototype.includes ( searchString [ ,
position ] )
This method performs the following steps when called:
If searchString appears as a substring of the result of
converting this object to a String, at one or more indices that are greater than or equal to
position, this function returns true; otherwise, it returns
false. If position is undefined, 0 is assumed, so as
to search all of the String.
Note 2
Throwing an exception if the first argument is a RegExp is specified in order to allow future
editions to define extensions that allow such argument values.
Note 3
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.9 String.prototype.indexOf ( searchString [ ,
position ] )
Note 1
If searchString appears as a substring of the result of
converting this object to a String, at one or more indices that are greater than or equal to
position, then the smallest such index is returned; otherwise,
-1𝔽 is returned. If position is
undefined, +0𝔽 is assumed, so as to search all of
the String.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.10 String.prototype.isWellFormed ( )
This method performs the following steps when called:
22.1.3.11 String.prototype.lastIndexOf ( searchString [ ,
position ] )
Note 1
If searchString appears as a substring of the result of
converting this object to a String at one or more indices that are smaller than or equal to
position, then the greatest such index is returned; otherwise,
-1𝔽 is returned. If position is
undefined, the length of the String value is assumed, so as to search all of the
String.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement this
method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the
ECMA-402 API the following specification of this method is used:
This method returns a Number other than NaN representing the result of an implementation-defined locale-sensitive String
comparison of the this value (converted to a String S) with that
(converted to a String thatValue). The result is intended to correspond with a sort order of String values
according to conventions of the host environment's current locale, and will be negative when
S is ordered before thatValue, positive when S is ordered after
thatValue, and zero in all other cases (representing no relative ordering between S
and thatValue).
Before performing the comparisons, this method performs the following steps to prepare the Strings:
The meaning of the optional second and third parameters to this method are defined in the ECMA-402
specification; implementations that do not include ECMA-402 support must not assign any other
interpretation to those parameter positions.
The actual return values are implementation-defined to permit encoding additional
information in them, but this method, when considered as a method of two arguments, is required to be a
consistent
comparator defining a total ordering on the set of all Strings. This method is also
required to recognize and honour canonical equivalence according to the Unicode Standard, including
returning +0𝔽 when comparing distinguishable Strings that are canonically
equivalent.
Note 1
This method itself is not directly suitable as an argument to Array.prototype.sort
because the latter requires a function of two arguments.
Note 2
This method may rely on whatever language- and/or locale-sensitive comparison functionality is
available to the ECMAScript environment from the host environment, and is intended to compare according to
the conventions of the host environment's current locale. However, regardless of
comparison capabilities, this method must recognize and honour canonical equivalence according to the
Unicode Standard—for example, the following comparisons must all return
+0𝔽:
// Å ANGSTROM SIGN vs.// Å LATIN CAPITAL LETTER A + COMBINING RING ABOVE"\u212B".localeCompare("A\u030A")
// Ω OHM SIGN vs.// Ω GREEK CAPITAL LETTER OMEGA"\u2126".localeCompare("\u03A9")
// ṩ LATIN SMALL LETTER S WITH DOT BELOW AND DOT ABOVE vs.// ṩ LATIN SMALL LETTER S + COMBINING DOT ABOVE + COMBINING DOT BELOW"\u1E69".localeCompare("s\u0307\u0323")
// ḍ̇ LATIN SMALL LETTER D WITH DOT ABOVE + COMBINING DOT BELOW vs.// ḍ̇ LATIN SMALL LETTER D WITH DOT BELOW + COMBINING DOT ABOVE"\u1E0B\u0323".localeCompare("\u1E0D\u0307")
// 가 HANGUL CHOSEONG KIYEOK + HANGUL JUNGSEONG A vs.// 가 HANGUL SYLLABLE GA"\u1100\u1161".localeCompare("\uAC00")
It is recommended that this method should not honour Unicode compatibility equivalents or
compatibility decompositions as defined in the Unicode Standard, chapter 3, section 3.7.
Note 3
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.13 String.prototype.match ( regexp )
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.14 String.prototype.matchAll ( regexp )
This method performs a regular expression match of the String representing the this
value against regexp and returns an iterator that yields match results. Each match result is
an Array containing the matched portion of the String as the first element, followed by the portions
matched by any capturing groups. If the regular expression never matches, the returned iterator
does not yield any match results.
This method is intentionally generic, it does not require that its
this value be a String object. Therefore, it can be transferred to other kinds of
objects for use as a method.
Note 2
Similarly to String.prototype.split,
String.prototype.matchAll is designed to typically act without mutating its inputs.
22.1.3.15 String.prototype.normalize ( [ form ] )
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be a
String object. Therefore it can be transferred to other kinds of objects for use as a method.
The abstract operation StringPad takes arguments S (a String), maxLength (a
non-negative integer),
fillString (a String), and placement (start or
end) and returns a String. It performs the following steps when called:
Let stringLength be the length of S.
If maxLength ≤ stringLength, return S.
If fillString is the empty String, return S.
Let fillLen be maxLength - stringLength.
Let truncatedStringFiller be the String value consisting of repeated concatenations of
fillString truncated to length fillLen.
If placement is start, return the string-concatenation of
truncatedStringFiller and S.
The argument maxLength will be clamped such that it can be no smaller than the length of
S.
Note 2
The argument fillString defaults to " " (the String value consisting
of the code unit 0x0020 SPACE).
22.1.3.17.3 ToZeroPaddedDecimalString ( n, minLength
)
The abstract operation ToZeroPaddedDecimalString takes arguments n (a non-negative integer) and minLength (a
non-negative integer) and
returns a String. It performs the following steps when called:
Let S be the String representation of n, formatted as a decimal number.
Return the String value that is made from n copies of S appended together.
Note 1
This method creates the String value consisting of the code units of the this
value (converted to String) repeated count times.
Note 2
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
The abstract operation GetSubstitution takes arguments matched (a String), str (a
String), position (a non-negative integer), captures (a List of either Strings or
undefined), namedCaptures (an Object or undefined), and
replacementTemplate (a String) and returns either a normal completion containing a
String or a throw completion. For the purposes
of this abstract operation, a decimal digit is a code unit in the inclusive
interval from 0x0030 (DIGIT ZERO) to 0x0039 (DIGIT NINE). It performs the following
steps when called:
Repeat, while templateRemainder is not the empty String,
NOTE: The following steps isolate ref (a prefix of
templateRemainder), determine refReplacement (its replacement), and then
append that replacement to result.
If templateRemainder starts with "$$", then
Let ref be "$$".
Let refReplacement be "$".
Else if templateRemainder starts with "$`", then
Let ref be "$`".
Let refReplacement be the substring of str from 0 to
position.
Else if templateRemainder starts with "$&", then
Let ref be "$&".
Let refReplacement be matched.
Else if templateRemainder starts with "$'" (0x0024 (DOLLAR SIGN)
followed by 0x0027 (APOSTROPHE)), then
Let ref be "$'".
Let matchLength be the length of matched.
Let tailPos be position + matchLength.
Let refReplacement be the substring of str from min(tailPos,
stringLength).
NOTE: tailPos can exceed stringLength only if this abstract
operation was invoked by a call to the intrinsic %Symbol.replace% method
of %RegExp.prototype%
on an object whose "exec" property is not the intrinsic
%RegExp.prototype.exec%.
Else if templateRemainder starts with "$" followed by 1 or more
decimal digits, then
If templateRemainder starts with "$" followed by 2 or more
decimal digits, let digitCount be 2; otherwise let digitCount be 1.
Let digits be the substring of templateRemainder from 1 to 1 +
digitCount.
Let captureLen be the number of elements in captures.
If index > captureLen and digitCount = 2, then
NOTE: When a two-digit replacement pattern specifies an index exceeding the count of
capturing groups, it is treated as a one-digit replacement pattern followed by a literal
digit.
Set digitCount to 1.
Set digits to the substring of digits from 0 to 1.
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.22 String.prototype.slice ( start, end )
This method returns a substring of the result of converting this object to a
String, starting from index start and running to, but not including, index end (or
through the end of the String if end is undefined). If start is
negative, it is treated as sourceLength + start
where sourceLength is the length of the String. If end is negative, it is treated as
sourceLength + end where sourceLength
is the length of the String. The result is a String value,
not a String object.
This method is intentionally generic; it does not require that its this value be a
String object. Therefore it can be transferred to other kinds of objects for use as a method.
This method returns an Array into which substrings of the result of converting this object to a String
have been stored. The substrings are determined by searching from left to right for occurrences of
separator; these occurrences are not part of any String in the returned array, but serve to
divide up the String value. The value of separator may be a String of any length or it may be
an object, such as a RegExp, that has a %Symbol.split% method.
The value of separator may be an empty String. In this case, separator does not
match the empty substring at the beginning or end of the input String, nor
does it match the empty substring at the end of the previous separator
match. If separator is the empty String, the String is split up into individual code unit
elements; the length of the result array equals the length of the String, and each
substring contains one code unit.
If the this value is (or converts to) the empty String, the result depends on
whether separator can match the empty String. If it can, the result array contains no
elements. Otherwise, the result array contains one element, which is the empty String.
If separator is undefined, then the result array contains just one
String, which is the this value (converted to a String). If limit is not
undefined, then the output array is truncated so that it contains no more than
limit elements.
Note 2
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.24 String.prototype.startsWith ( searchString [ ,
position ] )
This method performs the following steps when called:
If position is undefined, let pos be 0; else let
pos be ? ToIntegerOrInfinity(position).
Let start be the result of clampingpos between 0 and len.
Let searchLength be the length of searchStr.
If searchLength = 0, return true.
Let end be start + searchLength.
If end > len, return false.
Let substring be the substring of S from start to
end.
If substring is searchStr, return true.
Return false.
Note 1
This method returns true if the sequence of code units of searchString
converted to a String is the same as the corresponding code units of this object (converted to a
String) starting at index position. Otherwise it returns false.
Note 2
Throwing an exception if the first argument is a RegExp is specified in order to allow future
editions to define extensions that allow such argument values.
Note 3
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.25 String.prototype.substring ( start, end )
This method returns a substring of the result of converting this object to a
String, starting from index start and running to, but not including, index end of
the String (or through the end of the String if end is undefined). The
result is a String value, not a String
object.
If either argument is NaN or negative, it is replaced with zero; if either argument is
strictly greater than the length of the String, it is replaced with the length of the String.
If start is strictly greater than end, they are swapped.
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement this
method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the
ECMA-402 API the following specification of this method is used:
This method interprets a String value as a sequence of UTF-16 encoded code points, as described in
6.1.4.
It works exactly the same as toLowerCase except that it is intended to yield a
locale-sensitive result corresponding with conventions of the host environment's current locale. There will
only be a difference in the few cases (such as Turkish) where the rules for that language conflict with
the regular Unicode case mappings.
The meaning of the optional parameters to this method are defined in the ECMA-402 specification;
implementations that do not include ECMA-402 support must not use those parameter positions for anything
else.
Note
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement this
method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the
ECMA-402 API the following specification of this method is used:
This method interprets a String value as a sequence of UTF-16 encoded code points, as described in
6.1.4.
It works exactly the same as toUpperCase except that it is intended to yield a
locale-sensitive result corresponding with conventions of the host environment's current locale. There will
only be a difference in the few cases (such as Turkish) where the rules for that language conflict with
the regular Unicode case mappings.
The meaning of the optional parameters to this method are defined in the ECMA-402 specification;
implementations that do not include ECMA-402 support must not use those parameter positions for anything
else.
Note
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.28 String.prototype.toLowerCase ( )
This method interprets a String value as a sequence of UTF-16 encoded code points, as described in
6.1.4.
The result must be derived according to the locale-insensitive case mappings in the Unicode Character
Database (this explicitly includes not only the file UnicodeData.txt, but
also all locale-insensitive mappings in the file SpecialCasing.txt
that accompanies it).
Note 1
The case mapping of some code points may produce multiple code points. In this case the result String
may not be the same length as the source String. Because both toUpperCase and
toLowerCase have context-sensitive behaviour, the methods are not symmetrical. In other
words, s.toUpperCase().toLowerCase() is not necessarily equal to
s.toLowerCase().
Note 2
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.29 String.prototype.toString ( )
This method performs the following steps when called:
For a String object, this method happens to return the same thing as the valueOf method.
22.1.3.30 String.prototype.toUpperCase ( )
This method interprets a String value as a sequence of UTF-16 encoded code points, as described in
6.1.4.
It behaves in exactly the same way as String.prototype.toLowerCase, except that the String
is mapped using the toUppercase algorithm of the Unicode Default Case Conversion.
Note
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.32.1 TrimString ( string, where )
The abstract operation TrimString takes arguments string (an ECMAScript language value) and where
(start, end, or start+end) and
returns either a normal completion containing a
String or a throw completion. It interprets
string as a sequence of UTF-16 encoded code points, as described in 6.1.4. It performs the following
steps when called:
Let T be the String value that is a copy of S with both leading and
trailing white space removed.
Return T.
The definition of white space is the union of WhiteSpace and LineTerminator. When determining whether a Unicode code
point is in Unicode general category “Space_Separator” (“Zs”), code unit sequences are interpreted as
UTF-16 encoded code point sequences as specified in 6.1.4.
22.1.3.33 String.prototype.trimEnd ( )
This method interprets a String value as a sequence of UTF-16 encoded code points, as described in
6.1.4.
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.34 String.prototype.trimStart ( )
This method interprets a String value as a sequence of UTF-16 encoded code points, as described in
6.1.4.
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.35 String.prototype.valueOf ( )
This method performs the following steps when called:
The value of the "name" property of this method is
"[Symbol.iterator]".
22.1.4 Properties of String Instances
String instances are String exotic objects and have the internal methods
specified for such objects. String instances inherit properties from the String prototype object. String
instances also have a [[StringData]] internal slot. The [[StringData]] internal slot is the String value represented by this String object.
String instances have a "length" property, and a set of enumerable properties with
integer-indexed
names.
22.1.4.1 length
The number of elements in the String value represented by this String object.
Once a String object is initialized, this property is unchanging. It has the attributes { [[Writable]]: false, [[Enumerable]]:
false, [[Configurable]]: false }.
22.1.5 String Iterator Objects
A String
Iterator is an object that represents a specific iteration over some specific String instance
object. There is not a named constructor for String Iterator objects. Instead, String Iterator
objects are created by calling certain methods of String instance objects.
The initial value of the %Symbol.toStringTag% property is the String value
"String Iterator".
This property has the attributes { [[Writable]]: false,
[[Enumerable]]: false, [[Configurable]]: true }.
22.2 RegExp (Regular Expression) Objects
A RegExp object contains a regular expression and the associated flags.
Note
The form and functionality of regular expressions is modelled after the regular expression facility in
the Perl 5 programming language.
22.2.1 Patterns
The RegExp constructor
applies the following grammar to the input pattern String. An error occurs if the grammar cannot interpret
the String as an expansion of Pattern.
The abstract operation CountLeftCapturingParensWithin takes argument node (a Parse
Node) and returns a non-negative integer. It returns the number of left-capturing parentheses in
node. A left-capturing
parenthesis is any ( pattern character that is matched by the ( terminal
of the Atom::(GroupSpecifieroptDisjunction) production.
The abstract operation CountLeftCapturingParensBefore takes argument node (a Parse
Node) and returns a non-negative integer. It returns the number of left-capturing parentheses within the
enclosing pattern that occur to the left of node.
22.2.1.4 Static Semantics: MightBothParticipate ( x, y
)
The abstract operation MightBothParticipate takes arguments x (a Parse
Node) and y (a Parse Node) and returns a Boolean. It performs the
following steps when called:
The abstract operation GroupSpecifiersThatMatch takes argument thisGroupName (a GroupNameParse Node) and returns a List of GroupSpecifierParse Nodes. It performs the following
steps when called:
The syntax-directed operation
RegExpIdentifierCodePoints takes no arguments and returns a List of code points. It is defined
piecewise over the following productions:
The syntax-directed operation
RegExpIdentifierCodePoint takes no arguments and returns a code point. It is defined piecewise over the
following productions:
A regular expression pattern is converted into an Abstract Closure using the process described below. An
implementation is encouraged to use more efficient algorithms than the ones listed below, as long as the
results are the same. The Abstract Closure is used as the value of a RegExp object's
[[RegExpMatcher]] internal slot.
A Pattern is a BMP pattern if its associated
flags contain neither a u nor a v. Otherwise, it is a Unicode pattern. A BMP
pattern matches against a String interpreted as consisting of a sequence of 16-bit values that are Unicode
code points in the range of the Basic Multilingual Plane. A Unicode pattern matches against a String
interpreted as consisting of Unicode code points encoded using UTF-16. In the context of describing the
behaviour of a BMP pattern “character” means a single 16-bit Unicode BMP code point. In the context of
describing the behaviour of a Unicode pattern “character” means a UTF-16 encoded code point (6.1.4). In either context, “character
value” means the numeric value of the corresponding non-encoded code point.
The syntax and semantics of Pattern is defined
as if the source text for the Pattern was a
List of SourceCharacter values where each SourceCharacter corresponds to a Unicode code point. If a BMP
pattern contains a non-BMP SourceCharacter the entire pattern is encoded using UTF-16 and
the individual code units of that encoding are used as the elements of the List.
Note
For example, consider a pattern expressed in source text as the single non-BMP character U+1D11E
(MUSICAL SYMBOL G CLEF). Interpreted as a Unicode pattern, it would be a single element (character)
List consisting of the single code
point U+1D11E. However, interpreted as a BMP pattern, it is first UTF-16 encoded to produce a two
element List consisting of the code units
0xD834 and 0xDD1E.
Patterns are passed to the RegExp constructor as ECMAScript String values in which non-BMP
characters are UTF-16 encoded. For example, the single character MUSICAL SYMBOL G CLEF pattern,
expressed as a String value, is a String of
length 2 whose elements were the code units 0xD834 and 0xDD1E. So no further translation of the string
would be necessary to process it as a BMP pattern consisting of two pattern characters. However, to
process it as a Unicode pattern UTF16SurrogatePairToCodePoint must be used in
producing a List whose sole element is a single
pattern character, the code point U+1D11E.
An implementation may not actually perform such translations to or from UTF-16, but the semantics of
this specification requires that the result of pattern matching be as if such translations were
performed.
22.2.2.1 Notation
The descriptions below use the following internal data structures:
A CharSetElement is one of the two following entities:
If rer.[[UnicodeSets]] is false, then a
CharSetElement is a character in the sense of the Pattern Semantics above.
If rer.[[UnicodeSets]] is true, then a
CharSetElement is a sequence whose elements are characters in the sense of the Pattern Semantics
above. This includes the empty sequence, sequences of one character, and sequences of more than one
character. For convenience, when working with CharSetElements of this kind, an individual character
is treated interchangeably with a sequence of one character.
A CharSet is a mathematical set of
CharSetElements.
A CaptureRange is a
Record { [[StartIndex]], [[EndIndex]] } that represents the range
of characters included in a capture, where [[StartIndex]] is an integer representing the start index
(inclusive) of the range within Input, and [[EndIndex]] is an
integer representing the end
index (exclusive) of the range within Input. For any CaptureRange, these indices must
satisfy the invariant that [[StartIndex]] ≤ [[EndIndex]].
A MatchState is a Record { [[Input]], [[EndIndex]], [[Captures]] } where [[Input]] is a List of characters representing the
String being matched, [[EndIndex]] is an integer, and [[Captures]]
is a List of values, one for each left-capturing parenthesis in the pattern.
MatchStates are used to represent partial match states in
the regular expression matching algorithms. The [[EndIndex]] is one plus the
index of the last input character matched so far by the pattern, while [[Captures]] holds the results of capturing parentheses. The
nth element of [[Captures]] is either a CaptureRange representing the range of characters
captured by the nth set of capturing parentheses, or undefined
if the nth set of capturing parentheses hasn't been reached yet. Due to
backtracking, many MatchStates may be in use at any time during the matching
process.
A MatcherContinuation is an Abstract Closure that takes one
MatchState argument and returns either a MatchState or
failure. The MatcherContinuation attempts to match the
remaining portion (specified by the closure's captured values) of the pattern against Input,
starting at the intermediate state given by its MatchState argument. If the match succeeds, the MatcherContinuation returns the final MatchState
that it reached; if the match fails, the MatcherContinuation returns
failure.
A Matcher is an Abstract
Closure that takes two arguments—a MatchState and a MatcherContinuation—and returns either a
MatchState or failure. A Matcher attempts to
match a middle subpattern (specified by the closure's captured values) of the pattern against the
MatchState's [[Input]], starting
at the intermediate state given by its MatchState argument. The MatcherContinuation argument should be a closure
that matches the rest of the pattern. After matching the subpattern of a pattern to obtain a new
MatchState, the Matcher then calls MatcherContinuation on that new MatchState to
test if the rest of the pattern can match as well. If it can, the Matcher returns the MatchState
returned by MatcherContinuation; if not, the Matcher may try
different choices at its choice points, repeatedly calling MatcherContinuation until it
either succeeds or all possibilities have been exhausted.
22.2.2.1.1 RegExp Records
A RegExp Record is a Record value used to store information
about a RegExp that is needed during compilation and possibly during matching.
Let cap be a List of
rer.[[CapturingGroupsCount]]undefined
values, indexed 1 through rer.[[CapturingGroupsCount]].
Let x be the MatchState { [[Input]]:
Input, [[EndIndex]]: index, [[Captures]]: cap }.
Return m(x, c).
Note
A Pattern compiles to an Abstract Closure value. RegExpBuiltinExec can then apply this procedure to a
List of characters and an offset
within that List to determine whether the
pattern would match starting at exactly that offset within the List, and, if it does match, what
the values of the capturing parentheses would be. The algorithms in 22.2.2
are designed so that compiling a pattern may throw a SyntaxError exception; on the
other hand, once the pattern is successfully compiled, applying the resulting Abstract
Closure to find a match in a List of characters cannot throw an
exception (except for any implementation-defined exceptions that can occur
anywhere such as out-of-memory).
The | regular expression operator separates two alternatives. The pattern first tries to
match the left Alternative (followed
by the sequel of the regular expression); if it fails, it tries to match the right Disjunction (followed by the sequel of the
regular expression). If the left Alternative, the right Disjunction, and the sequel all have choice points, all
choices in the sequel are tried before moving on to the next choice in the left Alternative. If choices in the left Alternative are exhausted, the right
Disjunction is tried instead of the
left Alternative. Any capturing
parentheses inside a portion of the pattern skipped by | produce
undefined values instead of Strings. Thus, for example,
Consecutive Terms try to simultaneously
match consecutive portions of Input. When direction is
forward, if the left Alternative, the right Term, and the sequel of the regular expression all have choice
points, all choices in the sequel are tried before moving on to the next choice in the right Term, and all choices in the right Term are tried before moving on to the next choice
in the left Alternative. When
direction is backward, the evaluation order of Alternative and Term are reversed.
The abstract operation RepeatMatcher takes arguments m (a Matcher), min (a non-negative
integer), max (a
non-negative integer or +∞),
greedy (a Boolean), x (a MatchState), c (a MatcherContinuation), parenIndex (a
non-negative integer), and
parenCount (a non-negative integer) and returns either a MatchState or
failure. It performs the following steps when called:
If max = 0, return c(x).
Let d be a new MatcherContinuation with parameters
(y) that captures m, min, max, greedy,
x, c, parenIndex, and parenCount and performs the
following steps when called:
If min = 0 and y.[[EndIndex]] = x.[[EndIndex]], return
failure.
If min = 0, let min2 be 0; otherwise let min2 be
min - 1.
If max = +∞, let max2 be +∞; otherwise let max2 be
max - 1.
Return RepeatMatcher(m,
min2, max2, greedy, y, c,
parenIndex, parenCount).
Let cap be a copy of x.[[Captures]].
For each integerk in the inclusive interval from
parenIndex + 1 to parenIndex + parenCount, set
cap[k] to undefined.
Let Input be x.[[Input]].
Let e be x.[[EndIndex]].
Let xr be the MatchState { [[Input]]:
Input, [[EndIndex]]: e, [[Captures]]: cap }.
If min ≠ 0, return m(xr, d).
If greedy is false, then
Let z be c(x).
If z is not failure, return z.
Return m(xr, d).
Let z be m(xr, d).
If z is not failure, return z.
Return c(x).
Note 1
An Atom followed by a Quantifier is repeated the number of times
specified by the Quantifier. A
Quantifier can be non-greedy, in
which case the Atom pattern is repeated as
few times as possible while still matching the sequel, or it can be greedy, in which case the
Atom pattern is repeated as many times as
possible while still matching the sequel. The Atom pattern is repeated rather than the input character sequence
that it matches, so different repetitions of the Atom can match different input substrings.
Note 2
If the Atom and the sequel of the regular
expression all have choice points, the Atom is first matched as many (or as few, if non-greedy) times as
possible. All choices in the sequel are tried before moving on to the next choice in the last
repetition of Atom. All choices in the
last (nth) repetition of Atom
are tried before moving on to the next choice in the next-to-last (n - 1)st repetition of
Atom; at which point it may turn out that
more or fewer repetitions of Atom are now
possible; these are exhausted (again, starting with either as few or as many as possible) before
moving on to the next choice in the (n - 1)st repetition of Atom and so on.
Compare
/a[a-z]{2,4}/.exec("abcdefghi")
which returns "abcde" with
/a[a-z]{2,4}?/.exec("abcdefghi")
which returns "abc".
Consider also
/(aa|aabaac|ba|b|c)*/.exec("aabaac")
which, by the choice point ordering above, returns the array
["aaba", "ba"]
and not any of:
["aabaac", "aabaac"]
["aabaac", "c"]
The above ordering of choice points can be used to write a regular expression that calculates the
greatest common divisor of two numbers (represented in unary notation). The following example
calculates the gcd of 10 and 15:
Step 4 of the RepeatMatcher clears Atom's captures each time Atom is repeated. We can see its behaviour in
the regular expression
/(z)((a+)?(b+)?(c))*/.exec("zaacbbbcac")
which returns the array
["zaacbbbcac", "z", "ac", "a", undefined, "c"]
and not
["zaacbbbcac", "z", "ac", "a", "bbb", "c"]
because each iteration of the outermost * clears all captured Strings contained in the
quantified Atom, which in this case
includes capture Strings numbered 2, 3, 4, and 5.
Note 4
Step 2.b of the RepeatMatcher states that once the
minimum number of repetitions has been satisfied, any more expansions of Atom that match the empty character sequence are not considered
for further repetitions. This prevents the regular expression engine from falling into an infinite
loop on patterns such as:
/(a*)*/.exec("b")
or the slightly more complicated:
/(a*)b\1+/.exec("baaaac")
which returns the array
["b", ""]
22.2.2.3.2 EmptyMatcher ( )
The abstract operation EmptyMatcher takes no arguments and returns a Matcher. It performs the following steps
when called:
Return a new Matcher with parameters (x, c) that
captures nothing and performs the following steps when called:
The abstract operation MatchTwoAlternatives takes arguments m1 (a Matcher) and
m2 (a Matcher) and returns a Matcher. It performs the following steps
when called:
Return a new Matcher with parameters (x, c) that
captures m1 and m2 and performs the following steps when called:
The abstract operation MatchSequence takes arguments m1 (a Matcher), m2 (a Matcher), and
direction (forward or backward) and returns a
Matcher. It
performs the following steps when called:
If direction is forward, then
Return a new Matcher with parameters (x, c)
that captures m1 and m2 and performs the following steps when called:
If e = 0, or if rer.[[Multiline]] is
true and the character Input[e - 1] is matched by LineTerminator, then
Return c(x).
Return failure.
Note 2
Even when the y flag is used with a pattern, ^ always matches only at the
beginning of Input, or (if rer.[[Multiline]] is
true) at the beginning of a line.
Let z be the MatchState { [[Input]]:
Input, [[EndIndex]]: xe, [[Captures]]: cap }.
Return c(z).
Note 3
The form (?=Disjunction) specifies a zero-width positive
lookahead. In order for it to succeed, the pattern inside Disjunction must match at the current position, but the
current position is not advanced before matching the sequel. If Disjunction can match at the current position in several
ways, only the first one is tried. Unlike other regular expression operators, there is no backtracking
into a (?= form (this unusual behaviour is inherited from Perl). This only matters when
the Disjunction contains capturing
parentheses and the sequel of the pattern contains backreferences to those captures.
For example,
/(?=(a+))/.exec("baaabac")
matches the empty String immediately after the first b and therefore returns the array:
["", "aaa"]
To illustrate the lack of backtracking into the lookahead, consider:
The form (?!Disjunction) specifies a zero-width negative
lookahead. In order for it to succeed, the pattern inside Disjunction must fail to match at the current position. The
current position is not advanced before matching the sequel. Disjunction can contain capturing parentheses, but
backreferences to them only make sense from within Disjunction itself. Backreferences to these capturing
parentheses from elsewhere in the pattern always return undefined because the
negative lookahead must fail for the pattern to succeed. For example,
/(.*?)a(?!(a+)b\2c)\2(.*)/.exec("baaabaac")
looks for an a not immediately followed by some positive number n of a's, a
b, another n a's (specified by the first \2) and a
c. The second \2 is outside the negative lookahead, so it matches against
undefined and therefore always succeeds. The whole expression returns the array:
The abstract operation IsWordChar takes arguments rer (a RegExp
Record), Input (a List of characters),
and e (an integer) and returns a Boolean. It performs the following steps when
called:
Let InputLength be the number of elements in Input.
The syntax-directed operation
CompileQuantifier takes no arguments and returns a Record with fields [[Min]] (a non-negative integer), [[Max]] (a non-negative integer or +∞), and [[Greedy]] (a Boolean). It is defined piecewise over the following productions:
The syntax-directed operation
CompileQuantifierPrefix takes no arguments and returns a Record with fields [[Min]] (a non-negative integer) and [[Max]] (a non-negative integer or +∞). It is defined
piecewise over the following productions:
If rer.[[UnicodeSets]] is false, or if every
CharSetElement of cs consists of a single
character (including if cs is empty), return CharacterSetMatcher(rer,
cs, cc.[[Invert]], direction).
Let r be the CaptureRange { [[StartIndex]]: ye, [[EndIndex]]: xe }.
Set cap[parenIndex + 1] to r.
Let z be the MatchState { [[Input]]:
Input, [[EndIndex]]: ye, [[Captures]]: cap }.
Return c(z).
Return m(x, d).
Note 2
Parentheses of the form (Disjunction) serve both to group the
components of the Disjunction pattern
together and to save the result of the match. The result can be used either in a backreference
(\ followed by a non-zero decimal number), referenced in a replace String, or returned as
part of an array from the regular expression matching Abstract Closure. To inhibit the
capturing behaviour of parentheses, use the form (?:Disjunction) instead.
An escape sequence of the form \ followed by a non-zero decimal number n
matches the result of the nth set of capturing parentheses (22.2.2.1).
It is an error if the regular expression has fewer than n capturing parentheses. If the
regular expression has n or more capturing parentheses but the nth
one is undefined because it has not captured anything, then the backreference
always succeeds.
If rer.[[UnicodeSets]] is false, or if every
CharSetElement of cs consists of a single
character (including if cs is empty), return CharacterSetMatcher(rer,
cs, false, direction).
22.2.2.7.1 CharacterSetMatcher ( rer, A,
invert, direction )
The abstract operation CharacterSetMatcher takes arguments rer (a RegExp
Record), A (a CharSet), invert (a Boolean), and
direction (forward or backward) and returns a
Matcher. It
performs the following steps when called:
If there exists a CharSetElement in A containing
exactly one character a such that Canonicalize(rer,
a) is cc, let found be true; otherwise let
found be false.
If invert is false and found is
false, return failure.
If invert is true and found is
true, return failure.
Let cap be x.[[Captures]].
Let y be the MatchState { [[Input]]:
Input, [[EndIndex]]: f, [[Captures]]: cap }.
Return c(y).
22.2.2.7.2 BackreferenceMatcher ( rer, ns,
direction )
The abstract operation BackreferenceMatcher takes arguments rer (a RegExp
Record), ns (a List of positive
integers), and
direction (forward or backward) and returns a
Matcher. It
performs the following steps when called:
Return a new Matcher with parameters (x, c) that
captures rer, ns, and direction and performs the following steps
when called:
If there exists an integeri in the interval from 0 (inclusive) to
len (exclusive) such that Canonicalize(rer,
Input[rs + i]) is not Canonicalize(rer,
Input[g + i]), return failure.
Let y be the MatchState { [[Input]]:
Input, [[EndIndex]]: f, [[Captures]]: cap }.
Return c(y).
22.2.2.7.3 Canonicalize ( rer, ch )
The abstract operation Canonicalize takes arguments rer (a RegExp Record)
and ch (a character) and returns a character. It performs the following steps when called:
If the file CaseFolding.txt
of the Unicode Character Database provides a simple or common case folding mapping for
ch, return the result of applying that mapping to ch.
If the numeric value of ch ≥ 128 and the numeric value of cu < 128,
return ch.
Return cu.
Note
In case-insignificant matches when HasEitherUnicodeFlag(rer)
is true, all characters are implicitly case-folded using the simple mapping
provided by the Unicode Standard immediately before they are compared. The simple mapping always
maps to a single code point, so it does not map, for example, ß (U+00DF LATIN SMALL
LETTER SHARP S) to ss or SS. It may however map code points outside the
Basic Latin block to code points within it—for example, ſ (U+017F LATIN SMALL LETTER
LONG S) case-folds to s (U+0073 LATIN SMALL LETTER S) and K (U+212A KELVIN
SIGN) case-folds to k (U+006B LATIN SMALL LETTER K). Strings containing those code
points are matched by regular expressions such as /[a-z]/ui.
In case-insignificant matches when HasEitherUnicodeFlag(rer)
is false, the mapping is based on Unicode Default Case Conversion algorithm
toUppercase rather than toCasefold, which results in some subtle differences. For example,
Ω (U+2126 OHM SIGN) is mapped by toUppercase to itself but by toCasefold to
ω (U+03C9 GREEK SMALL LETTER OMEGA) along with Ω (U+03A9 GREEK CAPITAL
LETTER OMEGA), so "\u2126" is matched by /[ω]/ui and
/[\u03A9]/ui but not by /[ω]/i or /[\u03A9]/i. Also, no code
point outside the Basic Latin block is mapped to a code point within it, so strings such as
"\u017F ſ" and "\u212A K" are not matched by
/[a-z]/i.
22.2.2.7.4 UpdateModifiers ( rer, add,
remove )
The abstract operation UpdateModifiers takes arguments rer (a RegExp
Record), add (a String), and remove (a String) and returns a
RegExp
Record. It performs the following steps when called:
Assert: add
and remove have no elements in common.
Let ignoreCase be rer.[[IgnoreCase]].
Let multiline be rer.[[Multiline]].
Let dotAll be rer.[[DotAll]].
Let unicode be rer.[[Unicode]].
Let unicodeSets be rer.[[UnicodeSets]].
Let capturingGroupsCount be rer.[[CapturingGroupsCount]].
If remove contains "i", set ignoreCase to
false.
Else if add contains "i", set ignoreCase to
true.
If remove contains "m", set multiline to
false.
Else if add contains "m", set multiline to
true.
If remove contains "s", set dotAll to
false.
Else if add contains "s", set dotAll to
true.
Return the RegExp Record { [[IgnoreCase]]: ignoreCase, [[Multiline]]:
multiline, [[DotAll]]: dotAll, [[Unicode]]: unicode, [[UnicodeSets]]:
unicodeSets, [[CapturingGroupsCount]]:
capturingGroupsCount }.
22.2.2.8 Runtime Semantics: CompileCharacterClass
The syntax-directed operation
CompileCharacterClass takes argument rer (a RegExp Record) and returns a Record with fields [[CharSet]] (a CharSet) and [[Invert]] (a Boolean).
It is defined piecewise over the following productions:
ClassContents can expand into a
single ClassAtom and/or ranges of two
ClassAtom separated by dashes. In the
latter case the ClassContents
includes all characters between the first ClassAtom and the second ClassAtom, inclusive; an error occurs if either ClassAtom does not represent a single
character (for example, if one is \w) or if the first ClassAtom's character value is strictly greater than the
second ClassAtom's character value.
Note 3
Even if the pattern ignores case, the case of the two ends of a range is significant in determining
which characters belong to the range. Thus, for example, the pattern /[E-F]/i matches
only the letters E, F, e, and f, while the pattern
/[E-f]/i matches all uppercase and lowercase letters in the Unicode Basic Latin block as
well as the symbols [, \, ], ^, _,
and `.
Note 4
A - character can be treated literally or it can denote a range. It is treated literally
if it is the first or last character of ClassContents, the beginning or end limit of a range
specification, or immediately follows a range specification.
Let c be the character whose character value is cv.
Return the CharSet containing the single character c.
Note 5
A ClassAtom can use any of the escape
sequences that are allowed in the rest of the regular expression except for \b,
\B, and backreferences. Inside a CharacterClass, \b means the backspace
character, while \B and backreferences raise errors. Using a backreference inside a
ClassAtom causes an error.
Assert: p is a
binary Unicode property or binary property alias listed in the “Property
name and aliases” column of Table 68, or a binary
Unicode property of strings listed in the “Property name” column of
Table 69.
Let A be the CharSet containing all CharSetElements whose character
database definition includes the property p with value “True”.
The result will often consist of two or more ranges. When UnicodeSets is true and
IgnoreCase is true, then MaybeSimpleCaseFolding(rer, [Ā-č])
will include only the odd-numbered code points of that range.
Return the CharSet containing the single character U+0008 (BACKSPACE).
22.2.2.9.1 CharacterRange ( A, B )
The abstract operation CharacterRange takes arguments A (a CharSet) and B (a CharSet) and returns
a CharSet.
It performs the following steps when called:
Assert: A and
B each contain exactly one character.
Return the CharSet containing all characters with a character value
in the inclusive
interval from i to j.
22.2.2.9.2 HasEitherUnicodeFlag ( rer )
The abstract operation HasEitherUnicodeFlag takes argument rer (a RegExp Record)
and returns a Boolean. It performs the following steps when called:
If rer.[[Unicode]] is true or
rer.[[UnicodeSets]] is true, then
Return true.
Return false.
22.2.2.9.3 WordCharacters ( rer )
The abstract operation WordCharacters takes argument rer (a RegExp Record)
and returns a CharSet. Returns a CharSet containing the characters considered
"word characters" for the purposes of \b, \B, \w, and
\W It performs the following steps when called:
Let extraWordChars be the CharSet containing all characters c such that
c is not in basicWordChars but Canonicalize(rer,
c) is in basicWordChars.
Assert:
extraWordChars is empty unless HasEitherUnicodeFlag(rer)
is true and rer.[[IgnoreCase]] is
true.
Return the union of basicWordChars and extraWordChars.
22.2.2.9.4 AllCharacters ( rer )
The abstract operation AllCharacters takes argument rer (a RegExp Record)
and returns a CharSet. Returns the set of “all characters” according to the
regular expression flags. It performs the following steps when called:
If rer.[[UnicodeSets]] is true and
rer.[[IgnoreCase]] is true, then
Return the CharSet containing all Unicode code points
c that do not have a Simple Case Folding
mapping (that is, scf(c)=c).
Return the CharSet containing all code point values.
Else,
Return the CharSet containing all code unit values.
22.2.2.9.5 MaybeSimpleCaseFolding ( rer, A )
The abstract operation MaybeSimpleCaseFolding takes arguments rer (a RegExp Record)
and A (a CharSet) and returns a CharSet. If rer.[[UnicodeSets]] is false or rer.[[IgnoreCase]] is false, it returns A. Otherwise, it
uses the Simple Case Folding
(scf(cp)) definitions in the file
CaseFolding.txt of
the Unicode Character Database (each of which maps a single code point to another single code point) to
map each CharSetElement of A character-by-character
into a canonical form and returns the resulting CharSet. It performs the following steps when called:
If rer.[[UnicodeSets]] is false or
rer.[[IgnoreCase]] is false, return
A.
The abstract operation CharacterComplement takes arguments rer (a RegExp Record)
and S (a CharSet) and returns a CharSet. It performs the following steps
when called:
Return the CharSet containing the CharSetElements of A
which are not also CharSetElements of S.
22.2.2.9.7 UnicodeMatchProperty ( rer, p )
The abstract operation UnicodeMatchProperty takes arguments rer (a RegExp Record)
and p (ECMAScript source text) and returns a Unicode property name. It
performs the following steps when called:
If rer.[[UnicodeSets]] is true and
p is a Unicode property name listed in the
“Property name” column of Table 69, then
Implementations must support the Unicode property names and aliases listed in Table 67, Table
68, and Table 69. To ensure
interoperability, implementations must not support any other property names or aliases.
Note 1
For example, Script_Extensions (property name) and scx (property alias) are
valid, but script_extensions or Scx aren't.
Note 2
The listed properties form a superset of what UTS18 RL1.2 requires.
Note 3
The spellings of entries in these tables (including casing) match the spellings used in the file PropertyAliases.txt
in the Unicode Character Database. The precise spellings in that file are guaranteed to be
stable.
Table 67: Non-binary Unicode property aliases and their canonical property names
The abstract operation UnicodeMatchPropertyValue takes arguments p (ECMAScript source
text) and v (ECMAScript source text) and returns a Unicode property value.
It performs the following steps when called:
Implementations must support the Unicode property values and property value aliases listed in PropertyValueAliases.txt
for the properties listed in Table 67. To ensure interoperability,
implementations must not support any other property values or property value aliases.
Note 1
For example, Xpeo and Old_Persian are valid
Script_Extensions values, but xpeo and Old Persian aren't.
The syntax-directed operation
CompileClassSetString takes argument rer (a RegExp Record) and returns a sequence of
characters. It is defined piecewise over the following productions:
If F contains any code unit other than "d", "g",
"i", "m", "s", "u",
"v", or "y", or if F contains any code unit more than
once, throw a SyntaxError exception.
If F contains "i", let i be true; else
let i be false.
If F contains "m", let m be true; else
let m be false.
If F contains "s", let s be true; else
let s be false.
If F contains "u", let u be true; else
let u be false.
If F contains "v", let v be true; else
let v be false.
Let rer be the RegExp Record { [[IgnoreCase]]:
i, [[Multiline]]: m, [[DotAll]]: s, [[Unicode]]: u,
[[UnicodeSets]]: v, [[CapturingGroupsCount]]: capturingGroupsCount }.
Set obj.[[RegExpRecord]] to rer.
Set obj.[[RegExpMatcher]] to CompilePattern of
parseResult with argument rer.
22.2.3.4 Static Semantics: ParsePattern ( patternText,
u, v )
The abstract operation ParsePattern takes arguments patternText (a sequence of Unicode code
points), u (a Boolean), and v (a Boolean) and returns a Parse Node
or a non-empty List of SyntaxError
objects.
is the initial value of the "RegExp" property of the global object.
creates and initializes a new RegExp object when called as a constructor.
when called as a function rather than as a constructor, returns either a new RegExp object, or the argument
itself if the only argument is a RegExp object.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to
inherit the specified RegExp behaviour must include a super call to the RegExp constructor to create and
initialize subclass instances with the necessary internal slots.
22.2.4.1 RegExp ( pattern, flags )
This function performs the following steps when called:
If pattern is supplied using a StringLiteral, the usual escape sequence substitutions are
performed before the String is processed by this function. If pattern must contain an escape sequence
to be recognized by this function, any U+005C (REVERSE SOLIDUS) code points must be escaped within the
StringLiteral to prevent them being
removed when the contents of the StringLiteral are formed.
This function returns a copy of S in which characters that are potentially special in a
regular expression Pattern have been replaced
by equivalent escape sequences.
If escaped is the empty String and cp is matched by either DecimalDigit or AsciiLetter, then
NOTE: Escaping a leading digit ensures that output corresponds with pattern text which may
be used after a \0 character escape or a DecimalEscape such as \1 and still
match S rather than be interpreted as an extension of the preceding escape
sequence. Escaping a leading ASCII letter does the same for the context after \c.
Despite having similar names, EscapeRegExpPattern and RegExp.escape
do not perform similar actions. The former escapes a pattern for representation as a string, while
this function escapes a string for representation inside a pattern.
22.2.5.1.1 EncodeForRegExpEscape ( cp )
The abstract operation EncodeForRegExpEscape takes argument cp (a code point) and returns a
String. It returns a String representing a Pattern for matching cp. If cp is white
space or an ASCII punctuator, the returned value is an escape sequence. Otherwise, the returned value
is a String representation of
cp itself. It performs the following steps when called:
If cp is matched by SyntaxCharacter or cp is U+002F (SOLIDUS),
then
Else if cp is a code point listed in the “Code Point” column of Table 65, then
Return the string-concatenation of 0x005C (REVERSE SOLIDUS)
and the string in the “ControlEscape” column of the row whose “Code Point” column contains
cp.
Let otherPunctuators be the string-concatenation of
",-=<>#&!%:;@~'`" and the code unit 0x0022 (QUOTATION MARK).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
22.2.5.3 get RegExp [ %Symbol.species% ]
RegExp[%Symbol.species%] is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
Return the this value.
The value of the "name" property of this function is "get
[Symbol.species]".
Note
RegExp prototype methods normally use their this value's constructor to create a
derived object. However, a subclass constructor may over-ride that default behaviour by redefining
its %Symbol.species% property.
The RegExp prototype object does not have a "valueOf" property of its own; however,
it inherits the "valueOf" property from the Object prototype object.
22.2.6.1 RegExp.prototype.constructor
The initial value of RegExp.prototype.constructor is %RegExp%.
22.2.6.2 RegExp.prototype.exec ( string )
This method searches string for an occurrence of the regular expression pattern and returns an
Array containing the results of the match, or null if string did not match.
RegExp.prototype.dotAll is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
Let R be the this value.
Let cu be the code unit 0x0073 (LATIN SMALL LETTER S).
RegExp.prototype.flags is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
RegExp.prototype.global is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
Let R be the this value.
Let cu be the code unit 0x0067 (LATIN SMALL LETTER G).
RegExp.prototype.hasIndices is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
Let R be the this value.
Let cu be the code unit 0x0064 (LATIN SMALL LETTER D).
RegExp.prototype.ignoreCase is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
Let R be the this value.
Let cu be the code unit 0x0069 (LATIN SMALL LETTER I).
The value of the "name" property of this method is
"[Symbol.match]".
Note
The %Symbol.match% property is used by the IsRegExp abstract operation
to identify objects that have the basic behaviour of regular expressions. The absence of a %Symbol.match% property or the existence of such a
property whose value does not Boolean coerce to true indicates that the object is
not intended to be used as a regular expression object.
The value of the "name" property of this method is
"[Symbol.matchAll]".
22.2.6.10 get RegExp.prototype.multiline
RegExp.prototype.multiline is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
Let R be the this value.
Let cu be the code unit 0x006D (LATIN SMALL LETTER M).
NOTE: When n = 1, the preceding step puts the first element into
captures (at index 0). More generally, the nth capture (the
characters captured by the nth set of capturing parentheses) is at
captures[n - 1].
Let replacementString be ? GetSubstitution(matched, S,
position, captures, namedCaptures, replaceValue).
If position ≥ nextSourcePosition, then
NOTE: position should not normally move backwards. If it does, it is an
indication of an ill-behaving RegExp subclass or use of an access triggered side-effect to
change the global flag or other characteristics of rx. In such cases, the
corresponding substitution is ignored.
Set accumulatedResult to the string-concatenation of
accumulatedResult, the substring of S from
nextSourcePosition to position, and replacementString.
Set nextSourcePosition to position + matchLength.
If nextSourcePosition ≥ lengthS, return accumulatedResult.
The value of the "name" property of this method is
"[Symbol.search]".
Note
The "lastIndex" and "global" properties of this RegExp object
are ignored when performing the search. The "lastIndex" property is left unchanged.
22.2.6.13 get RegExp.prototype.source
RegExp.prototype.source is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
The abstract operation EscapeRegExpPattern takes arguments P (a String) and F (a
String) and returns a String. It performs the following steps when called:
If F contains "v", then
Let patternSymbol be Pattern[+UnicodeMode,
+UnicodeSetsMode].
Else if F contains "u", then
Let patternSymbol be Pattern[+UnicodeMode,
~UnicodeSetsMode].
Else,
Let patternSymbol be Pattern[~UnicodeMode,
~UnicodeSetsMode].
Let S be a String in the form of a patternSymbol equivalent to P
interpreted as UTF-16 encoded Unicode code points (6.1.4), in which certain code
points are escaped as described below. S may or may not differ from P;
however, the Abstract Closure that would result from evaluating
S as a patternSymbol must behave identically to the Abstract
Closure given by the constructed object's [[RegExpMatcher]] internal slot. Multiple calls to this abstract operation
using the same values for P and F must produce identical results.
The code points / or any LineTerminator occurring in the pattern shall be
escaped in S as necessary to ensure that the string-concatenation of
"/", S, "/", and F can be parsed (in an
appropriate lexical context) as a RegularExpressionLiteral that behaves
identically to the constructed regular expression. For example, if P is
"/", then S could be "\/" or
"\u002F", among other possibilities, but not "/", because
/// followed by F would be parsed as a SingleLineComment rather than a RegularExpressionLiteral. If
P is the empty String, this specification can be met by letting S be
"(?:)".
Return S.
Note
Despite having similar names, RegExp.escape and EscapeRegExpPattern do not perform
similar actions. The former escapes a string for representation inside a pattern, while this
function escapes a pattern for representation as a string.
This method returns an Array into which substrings of the result of converting string to a
String have been stored. The substrings are determined by searching from left to right for matches of
the this value regular expression; these occurrences are not part of any String in
the returned array, but serve to divide up the String value.
The this value may be an empty regular expression or a regular expression that can
match an empty String. In this case, the regular expression does not match the empty
substring at the beginning or end of the input String, nor does it match
the empty substring at the end of the previous separator match. (For
example, if the regular expression matches the empty String, the String is split up into individual
code unit elements; the length of the result array equals the length of the String, and each
substring contains one code unit.) Only the first match at a given index of
the String is considered, even if backtracking could yield a non-empty
substring match at that index. (For example,
/a*?/[Symbol.split]("ab") evaluates to the array ["a", "b"], while
/a*/[Symbol.split]("ab") evaluates to the array ["","b"].)
If string is (or converts to) the empty String, the result depends on whether the regular
expression can match the empty String. If it can, the result array contains no elements. Otherwise,
the result array contains one element, which is the empty String.
If the regular expression contains capturing parentheses, then each time separator is
matched the results (including any undefined results) of the capturing parentheses
are spliced into the output array. For example,
The value of the "name" property of this method is
"[Symbol.split]".
Note 2
This method ignores the value of the "global" and "sticky"
properties of this RegExp object.
22.2.6.15 get RegExp.prototype.sticky
RegExp.prototype.sticky is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
Let R be the this value.
Let cu be the code unit 0x0079 (LATIN SMALL LETTER Y).
The returned String has the form of a RegularExpressionLiteral that evaluates to
another RegExp object with the same behaviour as this object.
22.2.6.18 get RegExp.prototype.unicode
RegExp.prototype.unicode is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
Let R be the this value.
Let cu be the code unit 0x0075 (LATIN SMALL LETTER U).
RegExp.prototype.unicodeSets is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
Let R be the this value.
Let cu be the code unit 0x0076 (LATIN SMALL LETTER V).
The abstract operation RegExpExec takes arguments R (an Object) and S (a String)
and returns either a normal completion containing either an
Object or null, or a throw completion. It
performs the following steps when called:
If a callable "exec" property is not found this algorithm falls back to attempting
to use the built-in RegExp matching algorithm. This provides compatible behaviour for code written for
prior editions where most built-in algorithms that use regular expressions did not perform a dynamic
property lookup of "exec".
22.2.7.2 RegExpBuiltinExec ( R, S )
The abstract operation RegExpBuiltinExec takes arguments R (an initialized RegExp instance)
and S (a String) and returns either a normal completion containing either an
Array exotic
object or null, or a throw completion. It performs the
following steps when called:
Let length be the length of S.
Let lastIndex be ℝ(?
ToLength(? Get(R,
"lastIndex"))).
Let flags be R.[[OriginalFlags]].
If flags contains "g", let global be
true; else let global be false.
If flags contains "y", let sticky be
true; else let sticky be false.
If flags contains "d", let hasIndices be
true; else let hasIndices be false.
If global is false and sticky is false,
set lastIndex to 0.
Let matcher be R.[[RegExpMatcher]].
If flags contains "u" or flags contains
"v", let fullUnicode be true; else let
fullUnicode be false.
Let matchSucceeded be false.
If fullUnicode is true, let input be StringToCodePoints(S); otherwise let
input be a List whose elements are the code
units that are the elements of S.
NOTE: Each element of input is considered to be a character.
If capturedValue is not undefined, append s to
matchedGroupNames.
NOTE: If there are multiple groups named s, groups may already
have an s property at this point. However, because groups is an
ordinary
object whose properties are all writable data
properties, the call to CreateDataPropertyOrThrow is
nevertheless guaranteed to succeed.
The abstract operation AdvanceStringIndex takes arguments S (a String), index (a
non-negative integer), and
unicode (a Boolean) and returns an integer. It performs the following steps when called:
The abstract operation GetStringIndex takes arguments S (a String) and
codePointIndex (a non-negative integer) and returns a non-negative integer. It interprets S as a sequence of
UTF-16 encoded code points, as described in 6.1.4, and returns the
code unit index corresponding to code point index codePointIndex when such an index exists.
Otherwise, it returns the length of S. It performs the following steps when called:
If S is the empty String, return 0.
Let len be the length of S.
Let codeUnitCount be 0.
Let codePointCount be 0.
Repeat, while codeUnitCount < len,
If codePointCount = codePointIndex, return codeUnitCount.
The number of code units from the start of a string at which the match ends (exclusive).
22.2.7.6 GetMatchString ( S, match )
The abstract operation GetMatchString takes arguments S (a String) and match (a
Match
Record) and returns a String. It performs the following steps when called:
Assert:
match.[[StartIndex]] ≤ match.[[EndIndex]] ≤ the length of S.
Return the substring
of S from match.[[StartIndex]] to match.[[EndIndex]].
22.2.7.7 GetMatchIndexPair ( S, match )
The abstract operation GetMatchIndexPair takes arguments S (a String) and match (a
Match
Record) and returns an Array. It performs the following steps when called:
Assert:
match.[[StartIndex]] ≤ match.[[EndIndex]] ≤ the length of S.
The abstract operation MakeMatchIndicesIndexPairArray takes arguments S (a String),
indices (a List of either Match Records or
undefined), groupNames (a List of either Strings or
undefined), and hasGroups (a Boolean) and returns an Array. It performs the
following steps when called:
NOTE: If there are multiple groups named s, groups may already
have an s property at this point. However, because groups is an
ordinary
object whose properties are all writable data
properties, the call to CreateDataPropertyOrThrow is
nevertheless guaranteed to succeed.
RegExp instances are ordinary
objects that inherit properties from the RegExp prototype object. RegExp
instances have internal slots [[OriginalSource]], [[OriginalFlags]], [[RegExpRecord]], and [[RegExpMatcher]]. The value of the [[RegExpMatcher]]
internal slot is an Abstract Closure representation of the Pattern of the RegExp object.
Note
Prior to ECMAScript 2015, RegExp instances were specified as having the own data properties"source", "global", "ignoreCase", and
"multiline". Those properties are now specified as accessor properties of
RegExp.prototype.
RegExp instances also have the following property:
22.2.8.1 lastIndex
The value of the "lastIndex" property specifies the String index at which to start the
next match. It is coerced to an integral Number when used (see 22.2.7.2).
This property shall have the attributes { [[Writable]]: true,
[[Enumerable]]: false, [[Configurable]]: false }.
22.2.9 RegExp String Iterator Objects
A RegExp String Iterator is an object that represents a specific iteration over some
specific String instance object, matching against some specific RegExp instance object. There is not a named
constructor for RegExp
String Iterator objects. Instead, RegExp String Iterator objects are created by calling certain methods of
RegExp instance objects.
The abstract operation CreateRegExpStringIterator takes arguments R (an Object), S
(a String), global (a Boolean), and fullUnicode (a Boolean) and returns an Object.
It performs the following steps when called:
is the initial value of the "Array" property of the global object.
creates and initializes a new Array when called as a constructor.
also creates and initializes a new Array when called as a function rather than as a constructor. Thus the function
call Array(…) is equivalent to the object creation expression new Array(…) with
the same arguments.
is a function whose behaviour differs based upon the number and types of its arguments.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to
inherit the exotic Array behaviour must include a super call to the Array constructor to initialize
subclass instances that are Array exotic objects. However, most of the
Array.prototype methods are generic methods that are not dependent upon their
this value being an Array exotic object.
23.1.1.1 Array ( ...values )
This function performs the following steps when called:
If NewTarget is undefined, let newTarget be the active function
object; else let newTarget be NewTarget.
This method is an intentionally generic factory method; it does not require that its
this value be the Array constructor. Therefore it can be transferred to or inherited by
any other constructors that may be called with a single numeric argument.
23.1.2.2 Array.isArray ( arg )
This function performs the following steps when called:
This method is an intentionally generic factory method; it does not require that its
this value be the Array constructor. Therefore it can be transferred to or inherited by
other constructors
that may be called with a single numeric argument.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
23.1.2.5 get Array [ %Symbol.species% ]
Array[%Symbol.species%] is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
Return the this value.
The value of the "name" property of this function is "get
[Symbol.species]".
Note
Array prototype methods normally use their this value's constructor to create a
derived object. However, a subclass constructor may over-ride that default behaviour by redefining
its %Symbol.species% property.
23.1.3 Properties of the Array Prototype Object
The Array prototype object:
is %Array.prototype%.
is an Array exotic
object and has the internal methods specified for such objects.
has a "length" property whose initial value is +0𝔽 and
whose attributes are { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]:
false }.
The Array prototype object is specified to be an Array exotic object to ensure
compatibility with ECMAScript code that was created prior to the ECMAScript 2015 specification.
The explicit setting of the "length" property in step 6 is intended to ensure the length is
correct when the final non-empty element of items has trailing holes or when A
is not a built-in Array.
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
The initial value of Array.prototype.constructor is %Array%.
23.1.3.4 Array.prototype.copyWithin ( target, start [
, end ] )
Note 1
The end argument is optional. If it is not provided, the length of the
this value is used.
Note 2
If target is negative, it is treated as length +
target where length is the length of the array. If start
is negative, it is treated as length + start.
If end is negative, it is treated as length +
end.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.5 Array.prototype.entries ( )
This method performs the following steps when called:
callback should be a function that accepts three arguments and returns a value that is
coercible to a Boolean value. every calls callback once for each element
present in the array, in ascending order, until it finds one where callback returns
false. If such an element is found, every immediately returns
false. Otherwise, every returns true.
callback is called only for elements of the array which actually exist; it is not called
for missing elements of the array.
If a thisArg parameter is provided, it will be used as the this value
for each invocation of callback. If it is not provided, undefined is
used instead.
callback is called with three arguments: the value of the element, the index of the
element, and the object being traversed.
every does not directly mutate the object on which it is called but the object may be
mutated by the calls to callback.
The range of elements processed by every is set before the first call to
callback. Elements which are appended to the array after the call to every
begins will not be visited by callback. If existing elements of the array are changed,
their value as passed to callback will be the value at the time every visits
them; elements that are deleted after the call to every begins and before being visited
are not visited. every acts like the "for all" quantifier in mathematics. In particular,
for an empty array, it returns true.
This method performs the following steps when called:
Let testResult be ToBoolean(? Call(callback,
thisArg, « kValue, 𝔽(k), O »)).
If testResult is false, return false.
Set k to k + 1.
Return true.
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.7 Array.prototype.fill ( value [ , start [ ,
end ] ] )
Note 1
The start argument is optional. If it is not provided, +0𝔽
is used.
The end argument is optional. If it is not provided, the length of the
this value is used.
Note 2
If start is negative, it is treated as length +
start where length is the length of the array. If end is
negative, it is treated as length + end.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
callback should be a function that accepts three arguments and returns a value that is
coercible to a Boolean value. filter calls callback once for each element in
the array, in ascending order, and constructs a new array of all the values for which
callback returns true. callback is called only for elements
of the array which actually exist; it is not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as the this value
for each invocation of callback. If it is not provided, undefined is
used instead.
callback is called with three arguments: the value of the element, the index of the
element, and the object being traversed.
filter does not directly mutate the object on which it is called but the object may be
mutated by the calls to callback.
The range of elements processed by filter is set before the first call to
callback. Elements which are appended to the array after the call to filter
begins will not be visited by callback. If existing elements of the array are changed their
value as passed to callback will be the value at the time filter visits them;
elements that are deleted after the call to filter begins and before being visited are
not visited.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
This method calls predicate once for each element of the array, in ascending index order,
until it finds one where predicate returns a value that coerces to true.
If such an element is found, find immediately returns that element value. Otherwise,
find returns undefined.
Let findRec be ? FindViaPredicate(O,
len, ascending, predicate, thisArg).
Return findRec.[[Value]].
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
This method calls predicate once for each element of the array, in ascending index order,
until it finds one where predicate returns a value that coerces to true.
If such an element is found, findIndex immediately returns the index of that element
value. Otherwise, findIndex returns -1.
Let findRec be ? FindViaPredicate(O,
len, ascending, predicate, thisArg).
Return findRec.[[Index]].
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
This method calls predicate once for each element of the array, in descending index order,
until it finds one where predicate returns a value that coerces to true.
If such an element is found, findLast immediately returns that element value. Otherwise,
findLast returns undefined.
Let findRec be ? FindViaPredicate(O,
len, descending, predicate, thisArg).
Return findRec.[[Value]].
Note 2
This method is intentionally generic; it does not require that its this value be
an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
This method calls predicate once for each element of the array, in descending index order,
until it finds one where predicate returns a value that coerces to true.
If such an element is found, findLastIndex immediately returns the index of that element
value. Otherwise, findLastIndex returns -1.
Let findRec be ? FindViaPredicate(O,
len, descending, predicate, thisArg).
Return findRec.[[Index]].
Note 2
This method is intentionally generic; it does not require that its this value be
an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.12.1 FindViaPredicate ( O, len,
direction, predicate, thisArg )
O should be an array-like object or a TypedArray. This operation calls
predicate once for each element of O, in either ascending index order or
descending index order (as indicated by direction), until it finds one where
predicate returns a value that coerces to true. At that point, this
operation returns a Record that gives the index and value
of the element found. If no such element is found, this operation returns a Record that specifies
-1𝔽 for the index and undefined for the value.
predicate should be a function. When called for an element of the array, it is passed three
arguments: the value of the element, the index of the element, and the object being traversed. Its
return value will be coerced to a Boolean value.
thisArg will be used as the this value for each invocation of
predicate.
This operation does not directly mutate the object on which it is called, but the object may be mutated
by the calls to predicate.
The range of elements processed is set before the first call to predicate, just before the
traversal begins. Elements that are appended to the array after this will not be visited by
predicate. If existing elements of the array are changed, their value as passed to
predicate will be the value at the time that this operation visits them. Elements that are
deleted after traversal begins and before being visited are still visited and are either looked up from
the prototype or are undefined.
It performs the following steps when called:
If IsCallable(predicate) is
false, throw a TypeError exception.
If direction is ascending, then
Let indices be a List of the
integers in the
interval from 0
(inclusive) to len (exclusive), in ascending order.
Else,
Let indices be a List of the
integers in the
interval from 0
(inclusive) to len (exclusive), in descending order.
callback should be a function that accepts three arguments. forEach calls
callback once for each element present in the array, in ascending order.
callback is called only for elements of the array which actually exist; it is not called
for missing elements of the array.
If a thisArg parameter is provided, it will be used as the this value
for each invocation of callback. If it is not provided, undefined is
used instead.
callback is called with three arguments: the value of the element, the index of the
element, and the object being traversed.
forEach does not directly mutate the object on which it is called but the object may be
mutated by the calls to callback.
The range of elements processed by forEach is set before the first call to
callback. Elements which are appended to the array after the call to forEach
begins will not be visited by callback. If existing elements of the array are changed,
their value as passed to callback will be the value at the time forEach visits
them; elements that are deleted after the call to forEach begins and before being visited
are not visited.
This method performs the following steps when called:
Perform ? Call(callback, thisArg, «
kValue, 𝔽(k), O »).
Set k to k + 1.
Return undefined.
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
This method compares searchElement to the elements of the array, in ascending order, using
the SameValueZero algorithm, and if found at any position,
returns true; otherwise, it returns false.
The optional second argument fromIndex defaults to +0𝔽
(i.e. the whole array is searched). If it is greater than or equal to the length of the array,
false is returned, i.e. the array will not be searched. If it is less than
-0𝔽, it is used as the offset from the end of the array to compute
fromIndex. If the computed index is less than or equal to
+0𝔽, the whole array will be searched.
This method performs the following steps when called:
If SameValueZero(searchElement,
elementK) is true, return true.
Set k to k + 1.
Return false.
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
Note 3
This method intentionally differs from the similar indexOf method in two ways. First, it
uses the SameValueZero algorithm, instead of IsStrictlyEqual, allowing it to detect
NaN array elements. Second, it does not skip missing array elements, instead
treating them as undefined.
This method compares searchElement to the elements of the array, in ascending order, using the
IsStrictlyEqual algorithm, and if found at one or more
indices, returns the smallest such index; otherwise, it returns -1𝔽.
Note 1
The optional second argument fromIndex defaults to +0𝔽
(i.e. the whole array is searched). If it is greater than or equal to the length of the array,
-1𝔽 is returned, i.e. the array will not be searched. If it is less
than -0𝔽, it is used as the offset from the end of the array to compute
fromIndex. If the computed index is less than or equal to
+0𝔽, the whole array will be searched.
This method performs the following steps when called:
If IsStrictlyEqual(searchElement,
elementK) is true, return 𝔽(k).
Set k to k + 1.
Return -1𝔽.
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.18 Array.prototype.join ( separator )
This method converts the elements of the array to Strings, and then concatenates these Strings, separated
by occurrences of the separator. If no separator is provided, a single comma is used as the
separator.
This method is intentionally generic; it does not require that its this value be
an Array. Therefore, it can be transferred to other kinds of objects for use as a method.
23.1.3.19 Array.prototype.keys ( )
This method performs the following steps when called:
This method compares searchElement to the elements of the array in descending order using
the IsStrictlyEqual algorithm, and if found at one or more
indices, returns the largest such index; otherwise, it returns -1𝔽.
The optional second argument fromIndex defaults to the array's length minus one (i.e. the
whole array is searched). If it is greater than or equal to the length of the array, the whole array
will be searched. If it is less than -0𝔽, it is used as the offset from
the end of the array to compute fromIndex. If the computed index is less than or equal to
+0𝔽, -1𝔽 is returned.
This method performs the following steps when called:
If IsStrictlyEqual(searchElement,
elementK) is true, return 𝔽(k).
Set k to k - 1.
Return -1𝔽.
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
callback should be a function that accepts three arguments. map calls
callback once for each element in the array, in ascending order, and constructs a new Array
from the results. callback is called only for elements of the array which actually exist;
it is not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as the this value
for each invocation of callback. If it is not provided, undefined is
used instead.
callback is called with three arguments: the value of the element, the index of the
element, and the object being traversed.
map does not directly mutate the object on which it is called but the object may be
mutated by the calls to callback.
The range of elements processed by map is set before the first call to
callback. Elements which are appended to the array after the call to map
begins will not be visited by callback. If existing elements of the array are changed,
their value as passed to callback will be the value at the time map visits
them; elements that are deleted after the call to map begins and before being visited are
not visited.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.22 Array.prototype.pop ( )
Note 1
This method removes the last element of the array and returns it.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.23 Array.prototype.push ( ...items )
Note 1
This method appends the arguments to the end of the array, in the order in which they appear. It
returns the new length of the array.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
callback should be a function that takes four arguments. reduce calls the
callback, as a function, once for each element after the first element present in the array, in
ascending order.
callback is called with four arguments: the previousValue (value from the
previous call to callback), the currentValue (value of the current element), the
currentIndex, and the object being traversed. The first time that callback is called, the
previousValue and currentValue can be one of two values. If an
initialValue was supplied in the call to reduce, then previousValue
will be initialValue and currentValue will be the first value in the array. If
no initialValue was supplied, then previousValue will be the first value in the
array and currentValue will be the second. It is a TypeError if the
array contains no elements and initialValue is not provided.
reduce does not directly mutate the object on which it is called but the object may be
mutated by the calls to callback.
The range of elements processed by reduce is set before the first call to
callback. Elements that are appended to the array after the call to reduce
begins will not be visited by callback. If existing elements of the array are changed,
their value as passed to callback will be the value at the time reduce visits
them; elements that are deleted after the call to reduce begins and before being visited
are not visited.
This method performs the following steps when called:
Set accumulator to ? Call(callback,
undefined, « accumulator, kValue, 𝔽(k), O »).
Set k to k + 1.
Return accumulator.
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
callback should be a function that takes four arguments. reduceRight calls
the callback, as a function, once for each element after the first element present in the array, in
descending order.
callback is called with four arguments: the previousValue (value from the
previous call to callback), the currentValue (value of the current element), the
currentIndex, and the object being traversed. The first time the function is called, the
previousValue and currentValue can be one of two values. If an
initialValue was supplied in the call to reduceRight, then
previousValue will be initialValue and currentValue will be the last
value in the array. If no initialValue was supplied, then previousValue will be
the last value in the array and currentValue will be the second-to-last value. It is a
TypeError if the array contains no elements and initialValue is not
provided.
reduceRight does not directly mutate the object on which it is called but the object may
be mutated by the calls to callback.
The range of elements processed by reduceRight is set before the first call to
callback. Elements that are appended to the array after the call to
reduceRight begins will not be visited by callback. If existing elements of
the array are changed by callback, their value as passed to callback will be the
value at the time reduceRight visits them; elements that are deleted after the call to
reduceRight begins and before being visited are not visited.
This method performs the following steps when called:
Set accumulator to ? Call(callback,
undefined, « accumulator, kValue, 𝔽(k), O »).
Set k to k - 1.
Return accumulator.
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.26 Array.prototype.reverse ( )
Note 1
This method rearranges the elements of the array so as to reverse their order. It returns the
reversed array.
This method performs the following steps when called:
Assert:
lowerExists and upperExists are both false.
NOTE: No action is required.
Set lower to lower + 1.
Return O.
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore, it can be transferred to other kinds of objects for use as a method.
23.1.3.27 Array.prototype.shift ( )
This method removes the first element of the array and returns it.
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.28 Array.prototype.slice ( start, end )
This method returns an array containing the elements of the array from element start up to,
but not including, element end (or through the end of the array if end is
undefined). If start is negative, it is treated as length + start where length is the length of
the array. If end is negative, it is treated as length +
end where length is the length of the array.
The explicit setting of the "length" property in step 15 is intended to ensure the length is
correct even when A is not a built-in Array.
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
callback should be a function that accepts three arguments and returns a value that is
coercible to a Boolean value. some calls callback once for each element
present in the array, in ascending order, until it finds one where callback returns
true. If such an element is found, some immediately returns
true. Otherwise, some returns false.
callback is called only for elements of the array which actually exist; it is not called
for missing elements of the array.
If a thisArg parameter is provided, it will be used as the this value
for each invocation of callback. If it is not provided, undefined is
used instead.
callback is called with three arguments: the value of the element, the index of the
element, and the object being traversed.
some does not directly mutate the object on which it is called but the object may be
mutated by the calls to callback.
The range of elements processed by some is set before the first call to
callback. Elements that are appended to the array after the call to some
begins will not be visited by callback. If existing elements of the array are changed,
their value as passed to callback will be the value at the time that some
visits them; elements that are deleted after the call to some begins and before being
visited are not visited. some acts like the "exists" quantifier in mathematics. In
particular, for an empty array, it returns false.
This method performs the following steps when called:
Let testResult be ToBoolean(? Call(callback,
thisArg, « kValue, 𝔽(k), O »)).
If testResult is true, return true.
Set k to k + 1.
Return false.
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.30 Array.prototype.sort ( comparator )
This method sorts the elements of this array. If comparator is not
undefined, it should be a function that accepts two arguments x and
y and returns a negative Number if x < y, a positive Number if
x > y, or a zero otherwise.
It performs the following steps when called:
If comparator is not undefined and
IsCallable(comparator) is
false, throw a TypeError exception.
NOTE: The call to SortIndexedProperties in step 5 uses skip-holes.
The remaining indices are deleted to preserve the number of holes that were detected and excluded from
the sort.
Because non-existent property values always compare greater than undefined
property values, and undefined always compares greater than any other value (see
CompareArrayElements),
undefined property values always sort to the end of the result, followed by
non-existent property values.
This method is intentionally generic; it does not require that its this value be
an Array. Therefore, it can be transferred to other kinds of objects for use as a method.
There must be some mathematical permutation π of the non-negative integers less than itemCount, such that
for every non-negative integerj less than itemCount, the element
old[j] is exactly the same as new[π(j)].
Then for all non-negative integersj and k, each less than
itemCount, if ℝ(SortCompare(old[j], old[k])) <
0, then π(j) < π(k).
And for all non-negative integersj and k such that j <
k < itemCount, if ℝ(SortCompare(old[j],
old[k])) = 0, then π(j) <
π(k); i.e., the sort is stable.
Here the notation old[j] is used to refer to items[j] before step 4 is executed, and the notation new[j] to refer to items[j] after step 4 has been executed.
An abstract closure or function comparator is a consistent comparator for a set of values S if all of the requirements
below are met for all values a, b, and c (possibly the same value) in
the set S: The notation a <Cb means ℝ(comparator(a, b)) < 0;
a =Cb means ℝ(comparator(a, b)) = 0; and
a >Cb means ℝ(comparator(a, b)) > 0.
Calling comparator(a, b) always returns the same value v
when given a specific pair of values a and b as its two arguments. Furthermore,
vis a Number, and v is
not NaN. Note that this implies that exactly one of a <Cb, a =Cb, and a >Cb
will be true for a given pair of a and b.
Calling comparator(a, b) does not modify obj or any object
on obj's prototype chain.
a =Ca (reflexivity)
If a =Cb, then b =Ca (symmetry)
If a =Cb and b =Cc, then
a =Cc (transitivity of =C)
If a <Cb and b <Cc, then
a <Cc (transitivity of <C)
If a >Cb and b >Cc, then
a >Cc (transitivity of >C)
Note
The above conditions are necessary and sufficient to ensure that comparator divides the
set S into equivalence classes and that these equivalence classes are totally ordered.
23.1.3.30.2 CompareArrayElements ( x, y,
comparator )
This method deletes the deleteCount elements of the array starting at integer indexstart and replaces them with the elements of items. It returns an Array
containing the deleted elements (if any).
This method performs the following steps when called:
The explicit setting of the "length" property in steps 15 and 20 is intended to ensure the lengths
are correct even when the objects are not built-in Arrays.
Note 3
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement this
method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the
ECMA-402 API the following specification of this method is used.
Note 1
The first edition of ECMA-402 did not include a replacement specification for this method.
The meanings of the optional parameters to this method are defined in the ECMA-402 specification;
implementations that do not include ECMA-402 support must not use those parameter positions for anything
else.
This method performs the following steps when called:
This method converts the elements of the array to Strings using their toLocaleString
methods, and then concatenates these Strings, separated by occurrences of an implementation-defined locale-sensitive separator
String. This method is analogous to toString except that it is intended to yield a
locale-sensitive result corresponding with conventions of the host environment's current locale.
Note 3
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.33 Array.prototype.toReversed ( )
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.37 Array.prototype.unshift ( ...items )
This method prepends the arguments to the start of the array, such that their order within the array is
the same as the order in which they appear in the argument list.
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.38 Array.prototype.values ( )
This method performs the following steps when called:
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true }.
Note
The own property names of this object are property names that were not included as standard
properties of Array.prototype prior to the ECMAScript 2015 specification. These names are
ignored for with statement binding purposes in order to preserve the behaviour of
existing code that might use one of these names as a binding in an outer scope that is shadowed by a
with statement whose binding object is an Array.
The reason that "with" is not included in the unscopableList is because
it is already a reserved word.
Array instances have a "length" property, and a set of enumerable properties with
array index names.
23.1.4.1 length
The "length" property of an Array instance is a data property whose value is always
numerically greater than the name of every configurable own property whose name is an array index.
The "length" property initially has the attributes { [[Writable]]: true, [[Enumerable]]:
false, [[Configurable]]: false }.
Note
Reducing the value of the "length" property has the side-effect of deleting own
array elements whose array
index is between the old and new length values. However, non-configurable
properties can not be deleted. Attempting to set the "length" property of an Array
to a value that is numerically less than or equal to the largest numeric own property name of an
existing non-configurable array-indexed property of the array will result in the length
being set to a numeric value that is one greater than that non-configurable numeric own property name. See
10.4.2.1.
23.1.5 Array Iterator Objects
An Array
Iterator is an object that represents a specific iteration over some specific Array instance object.
There is not a named constructor for Array Iterator objects. Instead, Array Iterator
objects are created by calling certain methods of Array instance objects.
23.1.5.1 CreateArrayIterator ( array, kind )
The abstract operation CreateArrayIterator takes arguments array (an Object) and
kind (key+value, key, or
value) and returns an Object. It is used to create iterator
objects for Array methods that return such iterators. It performs the following
steps when called:
A value that identifies what is returned for each element of the iteration.
23.2 TypedArray Objects
A TypedArray presents an array-like view of an underlying binary data buffer (25.1). A
TypedArray element type is the underlying binary
scalar data type that all elements of a TypedArray instance have. There is a distinct
TypedArrayconstructor, listed in Table
73, for each of the supported element types. Each constructor in Table
73 has a corresponding distinct prototype object.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
23.2.2.4 get %TypedArray% [ %Symbol.species% ]
%TypedArray%[%Symbol.species%]
is an accessor
property whose set accessor function is undefined. Its get accessor
function performs the following steps when called:
Return the this value.
The value of the "name" property of this function is "get
[Symbol.species]".
%TypedArray%.prototype.buffer is
an accessor
property whose set accessor function is undefined. Its get accessor
function performs the following steps when called:
Assert: O has a
[[ViewedArrayBuffer]] internal slot.
Let buffer be O.[[ViewedArrayBuffer]].
Return buffer.
23.2.3.3 get %TypedArray%.prototype.byteLength
%TypedArray%.prototype.byteLength
is an accessor
property whose set accessor function is undefined. Its get accessor
function performs the following steps when called:
%TypedArray%.prototype.byteOffset
is an accessor
property whose set accessor function is undefined. Its get accessor
function performs the following steps when called:
If IsStrictlyEqual(searchElement,
elementK) is true, return 𝔽(k).
Set k to k - 1.
Return -1𝔽.
This method is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.
23.2.3.21 get %TypedArray%.prototype.length
%TypedArray%.prototype.length is
an accessor
property whose set accessor function is undefined. Its get accessor
function performs the following steps when called:
This method sets multiple values in this TypedArray, reading the values from
source. The details differ based upon the type of source. The optional
offset value indicates the first element index in this TypedArray where values are
written. If omitted, it is assumed to be 0.
The abstract operation SetTypedArrayFromArrayLike takes arguments target (a TypedArray),
targetOffset (a non-negative integer or +∞), and source (an ECMAScript language value, but not a TypedArray) and returns either
a normal completion containingunused or a throw completion.
It sets multiple values in target, starting at index targetOffset, reading the
values from source. It performs the following steps when called:
The abstract operation SetTypedArrayFromTypedArray takes arguments target (a TypedArray),
targetOffset (a non-negative integer or +∞), and source (a TypedArray) and returns either a normal completion containingunused or a throw completion.
It sets multiple values in target, starting at index targetOffset, reading the
values from source. It performs the following steps when called:
If targetOffset = +∞, throw a RangeError exception.
If srcLength + targetOffset > targetLength, throw a
RangeError exception.
If target.[[ContentType]] is not source.[[ContentType]], throw a TypeError exception.
If IsSharedArrayBuffer(srcBuffer) is
true, IsSharedArrayBuffer(targetBuffer) is
true, and srcBuffer.[[ArrayBufferData]] is
targetBuffer.[[ArrayBufferData]], let
sameSharedArrayBuffer be true; otherwise let
sameSharedArrayBuffer be false.
If SameValue(srcBuffer, targetBuffer) is
true or sameSharedArrayBuffer is true, then
This is a distinct method that, except as described below, implements the same requirements as those of
Array.prototype.sort as defined in 23.1.3.30. The implementation of this
method may be optimized with the knowledge that the this value is an object that has a
fixed length and whose integer-indexed properties are not sparse.
This method is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.
It performs the following steps when called:
If comparator is not undefined and IsCallable(comparator) is
false, throw a TypeError exception.
Because NaN always compares greater than any other value (see CompareTypedArrayElements),
NaN property values always sort to the end of the result when comparator
is not provided.
23.2.3.30 %TypedArray%.prototype.subarray ( start, end
)
This method returns a new TypedArray whose element type is the element type of this
TypedArray and whose ArrayBuffer is the ArrayBuffer of this TypedArray, referencing
the elements in the interval
from start (inclusive) to end (exclusive). If either start or
end is negative, it refers to an index from the end of the array, as opposed to from the
beginning.
This is a distinct method that implements the same algorithm as
Array.prototype.toLocaleString as defined in 23.1.3.32 except that TypedArrayLength
is called in place of performing a [[Get]] of "length". The
implementation of the algorithm may be optimized with the knowledge that the this value
has a fixed length when the underlying buffer is not resizable and whose integer-indexed properties are not sparse.
However, such optimization must not introduce any observable changes in the specified behaviour of the
algorithm.
This method is not generic. ValidateTypedArray is called with the
this value and seq-cst as arguments prior to evaluating the
algorithm. If its result is an abrupt completion
that exception is thrown instead of evaluating the algorithm.
Note
If the ECMAScript implementation includes the ECMA-402 Internationalization API this method is based
upon the algorithm for Array.prototype.toLocaleString that is in the ECMA-402
specification.
23.2.3.32 %TypedArray%.prototype.toReversed ( )
This method performs the following steps when called:
The initial value of the %Symbol.iterator% property is
%TypedArray.prototype.values%, defined in 23.2.3.35.
23.2.3.38 get %TypedArray%.prototype [ %Symbol.toStringTag% ]
%TypedArray%.prototype[%Symbol.toStringTag%]
is an accessor
property whose set accessor function is undefined. Its get accessor
function performs the following steps when called:
The abstract operation TypedArrayElementSize takes argument O (a TypedArray) and returns a non-negative integer. It performs the following
steps when called:
Return the Element Size value specified in Table 73 for
O.[[TypedArrayName]].
23.2.4.6 TypedArrayElementType ( O )
The abstract operation TypedArrayElementType takes argument O (a TypedArray) and returns a TypedArray element
type. It performs the following steps when called:
Return the Element Type value specified in Table 73 for
O.[[TypedArrayName]].
23.2.4.7 CompareTypedArrayElements ( x, y,
comparator )
The abstract operation CompareTypedArrayElements takes arguments x (a Number or a BigInt),
y (a Number or a BigInt), and comparator (a function object or
undefined) and returns either a normal completion containing a Number
or an abrupt completion. It performs the
following steps when called:
is an intrinsic object that has the structure described below, differing only in the name used as the
constructor name
instead of TypedArray, in Table 73.
is a function whose behaviour differs based upon the number and types of its arguments. The actual
behaviour of a call of TypedArray depends upon the number and kind of arguments that are passed
to it.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to
inherit the specified TypedArray behaviour must include a super call to the
TypedArrayconstructor to create and initialize the subclass instance with the
internal state necessary to support the %TypedArray%.prototype built-in
methods.
23.2.5.1TypedArray ( ...args )
Each TypedArrayconstructor performs the following steps when called:
If NewTarget is undefined, throw a TypeError exception.
Let constructorName be the String value of the Constructor Name value specified in Table 73 for this TypedArrayconstructor.
Let proto be "%TypedArray.prototype%".
Let numberOfArgs be the number of elements in args.
The abstract operation AllocateTypedArray takes arguments constructorName (a String which is
the name of a TypedArrayconstructor in Table 73), newTarget (a
constructor), and
defaultProto (a String) and optional argument length (a non-negative integer) and returns either a
normal completion containing a
TypedArray or a
throw completion. It is used to
validate and create an instance of a TypedArrayconstructor. If the length argument is passed, an
ArrayBuffer of that length is also allocated and associated with the new TypedArray instance. AllocateTypedArray provides
common semantics that is used by TypedArray. It performs the following steps when called:
23.2.5.1.5 InitializeTypedArrayFromArrayLike ( O,
arrayLike )
The abstract operation InitializeTypedArrayFromArrayLike takes arguments O (a TypedArray) and
arrayLike (an Object, but not a TypedArray or an ArrayBuffer) and returns either a normal completion containingunused or a throw completion.
It performs the following steps when called:
The abstract operation AllocateTypedArrayBuffer takes arguments O (a TypedArray) and
length (a non-negative integer) and returns either a normal completion containingunused or a throw completion.
It allocates and associates an ArrayBuffer with O. It performs the following steps when
called:
does not have a [[ViewedArrayBuffer]] or any other of the internal slots that
are specific to TypedArray instance objects.
23.2.7.1TypedArray.prototype.BYTES_PER_ELEMENT
The value of TypedArray.prototype.BYTES_PER_ELEMENT is the Element Size value
specified in Table 73 for TypedArray.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
23.2.7.2TypedArray.prototype.constructor
The initial value of the "constructor" property of the prototype for a given
TypedArrayconstructor is the constructor itself.
23.2.8 Properties of TypedArray Instances
TypedArray instances are TypedArrays. Each TypedArray instance inherits properties
from the corresponding TypedArray prototype object. Each TypedArray instance has the
following internal slots: [[ViewedArrayBuffer]], [[TypedArrayName]], [[ContentType]], [[ByteLength]], [[ByteOffset]], and [[ArrayLength]].
24 Keyed Collections
24.1 Map Objects
Maps are collections of key/value pairs where both the keys and values may be arbitrary ECMAScript
language values. A distinct key value may only occur in one key/value pair within the Map's
collection. Distinct key values are discriminated using the semantics of the SameValueZero comparison algorithm.
Maps must be implemented using either hash tables or other mechanisms that, on average, provide access times
that are sublinear on the number of elements in the collection. The data structure used in this specification
is only intended to describe the required observable semantics of Maps. It is not intended to be a viable
implementation model.
is the initial value of the "Map" property of the global object.
creates and initializes a new Map when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value in an extends clause of a class definition. Subclass constructors that intend to
inherit the specified Map behaviour must include a super call to the Map constructor to create and
initialize the subclass instance with the internal state necessary to support the
Map.prototype built-in methods.
24.1.1.1 Map ( [ iterable ] )
This function performs the following steps when called:
If NewTarget is undefined, throw a TypeError exception.
If the parameter iterable is present, it is expected to be an object that implements a
%Symbol.iterator% method that returns an iterator
object that produces a two element array-like object whose first
element is a value that will be used as a Map key and whose second element is the value to associate
with that key.
The parameter iterable is expected to be an object that implements a %Symbol.iterator% method that returns an iterator
object that produces a two element array-like object whose first
element is a value that will be used as a Map key and whose second element is the value to associate
with that key.
callback should be a function that accepts two arguments. groupBy calls
callback once for each element in items, in ascending order, and constructs a
new Map. Each value returned by callback is used as a key in the Map. For each such key,
the result Map has an entry whose key is that key and whose value is an array containing all the
elements for which callback returned that key.
callback is called with two arguments: the value of the element and the index of the
element.
The return value of groupBy is a Map.
This function performs the following steps when called:
Let groups be ? GroupBy(items, callback,
collection).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
24.1.2.3 get Map [ %Symbol.species% ]
Map[%Symbol.species%] is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
Return the this value.
The value of the "name" property of this function is "get
[Symbol.species]".
For each Record { [[Key]], [[Value]] } p of
M.[[MapData]], do
Set p.[[Key]] to empty.
Set p.[[Value]] to empty.
Return undefined.
Note
The existing [[MapData]]List is preserved because there may
be existing Map Iterator objects that are suspended midway
through iterating over that List.
24.1.3.2 Map.prototype.constructor
The initial value of Map.prototype.constructor is %Map%.
24.1.3.3 Map.prototype.delete ( key )
This method performs the following steps when called:
For each Record { [[Key]], [[Value]] } p of
M.[[MapData]], do
If p.[[Key]] is not empty and
SameValue(p.[[Key]],
key) is true, then
Set p.[[Key]] to empty.
Set p.[[Value]] to empty.
Return true.
Return false.
Note
The value empty is used as a specification device to indicate that an entry
has been deleted. Actual implementations may take other actions such as physically removing the entry
from internal data structures.
24.1.3.4 Map.prototype.entries ( )
This method performs the following steps when called:
If IsCallable(callback) is false,
throw a TypeError exception.
Let entries be M.[[MapData]].
Let numEntries be the number of elements in entries.
Let index be 0.
Repeat, while index < numEntries,
Let e be entries[index].
Set index to index + 1.
If e.[[Key]] is not empty, then
Perform ? Call(callback, thisArg, «
e.[[Value]], e.[[Key]], M »).
NOTE: The number of elements in entries may have increased during execution of
callback.
Set numEntries to the number of elements in entries.
Return undefined.
Note
callback should be a function that accepts three arguments. forEach calls
callback once for each key/value pair present in the Map, in key insertion order.
callback is called only for keys of the Map which actually exist; it is not called for keys
that have been deleted from the Map.
If a thisArg parameter is provided, it will be used as the this value
for each invocation of callback. If it is not provided, undefined is
used instead.
callback is called with three arguments: the value of the item, the key of the item, and
the Map being traversed.
forEach does not directly mutate the object on which it is called but the object may be
mutated by the calls to callback. Each entry of a map's [[MapData]] is only visited once. New keys added after the call to
forEach begins are visited. A key will be revisited if it is deleted after it has been
visited and then re-added before the forEach call completes. Keys that are deleted after
the call to forEach begins and before being visited are not visited unless the key is
added again before the forEach call completes.
24.1.3.6 Map.prototype.get ( key )
This method performs the following steps when called:
For each Record { [[Key]], [[Value]] } p of
M.[[MapData]], do
If p.[[Key]] is not empty and
SameValue(p.[[Key]],
key) is true, then
Set p.[[Value]] to value.
Return M.
Let p be the Record { [[Key]]: key, [[Value]]:
value }.
Append p to M.[[MapData]].
Return M.
24.1.3.10 get Map.prototype.size
Map.prototype.size is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
A Map Iterator
is an object that represents a specific iteration over some specific Map instance object. There is not a
named constructor for
Map Iterator objects. Instead, Map Iterator objects are created by calling certain methods of Map instance
objects.
The initial value of the %Symbol.toStringTag% property is the String value
"Map Iterator".
This property has the attributes { [[Writable]]: false,
[[Enumerable]]: false, [[Configurable]]: true }.
24.2 Set Objects
Set objects are collections of ECMAScript language values. A distinct value may only
occur once as an element of a Set's collection. Distinct values are discriminated using the semantics of the
SameValueZero
comparison algorithm.
Set objects must be implemented using either hash tables or other mechanisms that, on average, provide access
times that are sublinear on the number of elements in the collection. The data structure used in this
specification is only intended to describe the required observable semantics of Set objects. It is not
intended to be a viable implementation model.
24.2.1 Abstract Operations For Set Objects
24.2.1.1 Set Records
A Set Record is a Record value used to encapsulate the
interface of a Set or similar object.
If IsCallable(keys) is false,
throw a TypeError exception.
Return a new Set
Record { [[SetObject]]: obj, [[Size]]: intSize, [[Has]]: has,
[[Keys]]: keys }.
24.2.1.3 SetDataHas ( setData, value )
The abstract operation SetDataHas takes arguments setData (a List of either ECMAScript language values or
empty) and value (an ECMAScript language value) and
returns a Boolean. It performs the following steps when called:
If SetDataIndex(setData, value) is
not-found, return false.
Return true.
24.2.1.4 SetDataIndex ( setData, value )
The abstract operation SetDataIndex takes arguments setData (a List of either ECMAScript language values or
empty) and value (an ECMAScript language value) and
returns a non-negative integer
or not-found. It performs the following steps when called:
The abstract operation SetDataSize takes argument setData (a List of either ECMAScript language values or
empty) and returns a non-negative integer. It performs the following steps when called:
is the initial value of the "Set" property of the global object.
creates and initializes a new Set object when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value in an extends clause of a class definition. Subclass constructors that intend to
inherit the specified Set behaviour must include a super call to the Set constructor to create and
initialize the subclass instance with the internal state necessary to support the
Set.prototype built-in methods.
24.2.2.1 Set ( [ iterable ] )
This function performs the following steps when called:
If NewTarget is undefined, throw a TypeError exception.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
24.2.3.2 get Set [ %Symbol.species% ]
Set[%Symbol.species%] is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
Return the this value.
The value of the "name" property of this function is "get
[Symbol.species]".
Replace the element of S.[[SetData]] whose value is
e with an element whose value is empty.
Return undefined.
Note
The existing [[SetData]]List is preserved because there may
be existing Set Iterator objects that are suspended midway
through iterating over that List.
24.2.4.3 Set.prototype.constructor
The initial value of Set.prototype.constructor is %Set%.
24.2.4.4 Set.prototype.delete ( value )
This method performs the following steps when called:
If e is not empty and SameValue(e,
value) is true, then
Replace the element of S.[[SetData]] whose value is
e with an element whose value is empty.
Return true.
Return false.
Note
The value empty is used as a specification device to indicate that an entry
has been deleted. Actual implementations may take other actions such as physically removing the entry
from internal data structures.
24.2.4.5 Set.prototype.difference ( other )
This method performs the following steps when called:
NOTE: The number of elements in entries may have increased during execution of
callback.
Set numEntries to the number of elements in entries.
Return undefined.
Note
callback should be a function that accepts three arguments. forEach calls
callback once for each value present in the Set object, in value insertion order.
callback is called only for values of the Set which actually exist; it is not called for
keys that have been deleted from the set.
If a thisArg parameter is provided, it will be used as the this value
for each invocation of callback. If it is not provided, undefined is
used instead.
callback is called with three arguments: the first two arguments are a value contained in
the Set. The same value is passed for both arguments. The Set object being traversed is passed as the
third argument.
The callback is called with three arguments to be consistent with the call back functions
used by forEach methods for Map and Array. For Sets, each item value is considered to be
both the key and the value.
forEach does not directly mutate the object on which it is called but the object may be
mutated by the calls to callback.
Each value is normally visited only once. However, a value will be revisited if it is deleted after
it has been visited and then re-added before the forEach call completes. Values that are
deleted after the call to forEach begins and before being visited are not visited unless
the value is added again before the forEach call completes. New values added after the
call to forEach begins are visited.
24.2.4.8 Set.prototype.has ( value )
This method performs the following steps when called:
If SetDataSize(O.[[SetData]]) ≤ otherRec.[[Size]], then
Let thisSize be the number of elements in O.[[SetData]].
Let index be 0.
Repeat, while index < thisSize,
Let e be O.[[SetData]][index].
Set index to index + 1.
If e is not empty, then
Let inOther be ToBoolean(? Call(otherRec.[[Has]], otherRec.[[SetObject]],
« e »)).
If inOther is true, then
NOTE: It is possible for earlier calls to otherRec.[[Has]] to remove and re-add an element of O.[[SetData]], which can cause the same element to be visited
twice during this iteration.
The initial value of the "keys" property is %Set.prototype.values%, defined in
24.2.4.17.
Note
For iteration purposes, a Set appears similar to a Map where each entry has the same value for its
key and value.
24.2.4.14 get Set.prototype.size
Set.prototype.size is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
A Set Iterator
is an ordinary
object, with the structure defined below, that represents a specific iteration over some
specific Set instance object. There is not a named constructor for Set Iterator objects. Instead, Set Iterator objects
are created by calling certain methods of Set instance objects.
The initial value of the %Symbol.toStringTag% property is the String value
"Set Iterator".
This property has the attributes { [[Writable]]: false,
[[Enumerable]]: false, [[Configurable]]: true }.
24.3 WeakMap Objects
WeakMaps are collections of key/value pairs where the keys are objects and/or symbols and values may be
arbitrary ECMAScript language values. A WeakMap may be queried
to see if it contains a key/value pair with a specific key, but no mechanism is provided for enumerating the
values it holds as keys. In certain conditions, values which are not live are removed as WeakMap keys, as described in
9.9.3.
An implementation may impose an arbitrarily determined latency between the time a key/value pair of a WeakMap
becomes inaccessible and the time when the key/value pair is removed from the WeakMap. If this latency was
observable to ECMAScript program, it would be a source of indeterminacy that could impact program execution.
For that reason, an ECMAScript implementation must not provide any means to observe a key of a WeakMap that
does not require the observer to present the observed key.
WeakMaps must be implemented using either hash tables or other mechanisms that, on average, provide access
times that are sublinear on the number of key/value pairs in the collection. The data structure used in this
specification is only intended to describe the required observable semantics of WeakMaps. It is not intended
to be a viable implementation model.
Note
WeakMap and WeakSet are intended to provide mechanisms for dynamically associating state with an object
or symbol in a manner that does not “leak” memory resources if, in the absence of the WeakMap or WeakSet
instance, the object or symbol otherwise became inaccessible and subject to resource reclamation by the
implementation's garbage collection mechanisms. This characteristic can be achieved by using an inverted
per-object/symbol mapping of WeakMap or WeakSet instances to keys. Alternatively, each WeakMap or WeakSet
instance may internally store its key and value data, but this approach requires coordination between the
WeakMap or WeakSet implementation and the garbage collector. The following references describe mechanism
that may be useful to implementations of WeakMap and WeakSet:
Barry Hayes. 1997. Ephemerons: a new finalization mechanism. In Proceedings of the 12th ACM SIGPLAN
conference on Object-oriented programming, systems, languages, and applications (OOPSLA '97), A.
Michael Berman (Ed.). ACM, New York, NY, USA, 176-183, http://doi.acm.org/10.1145/263698.263733.
is the initial value of the "WeakMap" property of the global object.
creates and initializes a new WeakMap when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value in an extends clause of a class definition. Subclass constructors that intend to
inherit the specified WeakMap behaviour must include a super call to the WeakMap constructor to create and
initialize the subclass instance with the internal state necessary to support the
WeakMap.prototype built-in methods.
24.3.1.1 WeakMap ( [ iterable ] )
This function performs the following steps when called:
If NewTarget is undefined, throw a TypeError exception.
If the parameter iterable is present, it is expected to be an object that implements a
%Symbol.iterator% method that returns an iterator
object that produces a two element array-like object whose first
element is a value that will be used as a WeakMap key and whose second element is the value to
associate with that key.
For each Record { [[Key]], [[Value]] } p of
M.[[WeakMapData]], do
If p.[[Key]] is not empty and
SameValue(p.[[Key]],
key) is true, then
Set p.[[Key]] to empty.
Set p.[[Value]] to empty.
Return true.
Return false.
Note
The value empty is used as a specification device to indicate that an entry
has been deleted. Actual implementations may take other actions such as physically removing the entry
from internal data structures.
24.3.3.3 WeakMap.prototype.get ( key )
This method performs the following steps when called:
WeakSets are collections of objects and/or symbols. A distinct object or symbol may only occur once as an
element of a WeakSet's collection. A WeakSet may be queried to see if it contains a specific value, but no
mechanism is provided for enumerating the values it holds. In certain conditions, values which are not
live are removed as
WeakSet elements, as described in 9.9.3.
An implementation may impose an arbitrarily determined latency between the time a value contained in a
WeakSet becomes inaccessible and the time when the value is removed from the WeakSet. If this latency was
observable to ECMAScript program, it would be a source of indeterminacy that could impact program execution.
For that reason, an ECMAScript implementation must not provide any means to determine if a WeakSet contains a
particular value that does not require the observer to present the observed value.
WeakSets must be implemented using either hash tables or other mechanisms that, on average, provide access
times that are sublinear on the number of elements in the collection. The data structure used in this
specification is only intended to describe the required observable semantics of WeakSets. It is not intended
to be a viable implementation model.
is the initial value of the "WeakSet" property of the global object.
creates and initializes a new WeakSet when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value in an extends clause of a class definition. Subclass constructors that intend to
inherit the specified WeakSet behaviour must include a super call to the WeakSet constructor to create and
initialize the subclass instance with the internal state necessary to support the
WeakSet.prototype built-in methods.
24.4.1.1 WeakSet ( [ iterable ] )
This function performs the following steps when called:
If NewTarget is undefined, throw a TypeError exception.
If e is not empty and SameValue(e,
value) is true, then
Replace the element of S.[[WeakSetData]] whose value is
e with an element whose value is empty.
Return true.
Return false.
Note
The value empty is used as a specification device to indicate that an entry
has been deleted. Actual implementations may take other actions such as physically removing the entry
from internal data structures.
24.4.3.4 WeakSet.prototype.has ( value )
This method performs the following steps when called:
The descriptions below in this section, 25.4, and 29 use the read-modify-write modification function internal data
structure.
A read-modify-write modification
function is a mathematical function that is represented as an abstract closure that takes two
Lists of byte values as arguments and returns a List of byte values. These abstract closures satisfy all
of the following properties:
They perform all their algorithm steps atomically.
Their individual algorithm steps are not observable.
Note
To aid verifying that a read-modify-write modification function's algorithm steps constitute a pure,
mathematical function, the following editorial conventions are recommended:
They do not access, directly or transitively via invoked abstract operations and
abstract closures, any language or specification values except their parameters and captured values.
The abstract operation AllocateArrayBuffer takes arguments constructor (a constructor) and
byteLength (a non-negative integer) and optional argument maxByteLength (a non-negative
integer or
empty) and returns either a normal completion containing an
ArrayBuffer or a throw completion. It is used to create
an ArrayBuffer. It performs the following steps when called:
Let slots be « [[ArrayBufferData]], [[ArrayBufferByteLength]], [[ArrayBufferDetachKey]] ».
If maxByteLength is present and maxByteLength is not
empty, let allocatingResizableBuffer be true;
otherwise let allocatingResizableBuffer be false.
If allocatingResizableBuffer is true, then
If byteLength > maxByteLength, throw a RangeError
exception.
If it is not possible to create a Data Blockblock consisting of
maxByteLength bytes, throw a RangeError exception.
NOTE: Resizable ArrayBuffers are designed to be implementable with in-place growth.
Implementations may throw if, for example, virtual memory cannot be reserved up front.
Set obj.[[ArrayBufferMaxByteLength]] to
maxByteLength.
Return obj.
25.1.3.2 ArrayBufferByteLength ( arrayBuffer, order )
The abstract operation ArrayBufferByteLength takes arguments arrayBuffer (an ArrayBuffer or
SharedArrayBuffer) and order (seq-cst or
unordered) and returns a non-negative integer. It performs the following steps when called:
If IsSharedArrayBuffer(arrayBuffer) is
true and arrayBuffer has an [[ArrayBufferByteLengthData]] internal slot, then
Let bufferByteLengthBlock be arrayBuffer.[[ArrayBufferByteLengthData]].
NOTE: Neither creation of the new Data Block nor copying from the old Data Block are
observable. Implementations may implement this method as a zero-copy move or a realloc.
The abstract operation IsDetachedBuffer takes argument arrayBuffer (an ArrayBuffer or a
SharedArrayBuffer) and returns a Boolean. It performs the following steps when called:
If arrayBuffer.[[ArrayBufferData]] is null,
return true.
The abstract operation DetachArrayBuffer takes argument arrayBuffer (an ArrayBuffer) and
optional argument key (anything) and returns either a normal completion containingunused or a throw completion. It
performs the following steps when called:
If arrayBuffer.[[ArrayBufferDetachKey]] is not key,
throw a TypeError exception.
Set arrayBuffer.[[ArrayBufferData]] to null.
Set arrayBuffer.[[ArrayBufferByteLength]] to 0.
Return unused.
Note
Detaching an ArrayBuffer instance disassociates the Data Block used as its backing store from
the instance and sets the byte length of the buffer to 0.
The abstract operation CloneArrayBuffer takes arguments srcBuffer (an ArrayBuffer or a
SharedArrayBuffer), srcByteOffset (a non-negative integer), and srcLength (a non-negative integer) and returns either a
normal completion containing an
ArrayBuffer or a throw completion. It creates a new
ArrayBuffer whose data is a copy of srcBuffer's data over the range starting at
srcByteOffset and continuing for srcLength bytes. It performs the following steps
when called:
The host-defined
abstract operation HostResizeArrayBuffer takes arguments buffer (an ArrayBuffer) and
newByteLength (a non-negative integer) and returns either a normal completion containing either
handled or unhandled, or a throw completion. It gives the
host an opportunity to perform
implementation-defined resizing of buffer. If
the host chooses not to handle
resizing of buffer, it may return unhandled for the default behaviour.
The implementation of HostResizeArrayBuffer must conform to the following requirements:
The abstract operation does not detach buffer.
If the abstract operation completes normally with handled,
buffer.[[ArrayBufferByteLength]] is newByteLength.
The default implementation of HostResizeArrayBuffer is to return NormalCompletion(unhandled).
25.1.3.9 IsFixedLengthArrayBuffer ( arrayBuffer )
The abstract operation IsFixedLengthArrayBuffer takes argument arrayBuffer (an ArrayBuffer or
a SharedArrayBuffer) and returns a Boolean. It performs the following steps when called:
If arrayBuffer has an [[ArrayBufferMaxByteLength]] internal
slot, return false.
Return true.
25.1.3.10 IsUnsignedElementType ( type )
The abstract operation IsUnsignedElementType takes argument type (a TypedArray element
type) and returns a Boolean. It verifies if the argument type is an unsigned
TypedArray
element type. It performs the following steps when called:
If type is one of uint8, uint8clamped,
uint16, uint32, or biguint64,
return true.
Return false.
25.1.3.11 IsUnclampedIntegerElementType ( type )
The abstract operation IsUnclampedIntegerElementType takes argument type (a TypedArray element
type) and returns a Boolean. It verifies if the argument type is an
IntegerTypedArray element
type not including uint8clamped. It performs the following steps
when called:
If type is one of int8, uint8,
int16, uint16, int32, or
uint32, return true.
If type is either biguint64 or bigint64,
return true.
Return false.
25.1.3.13 IsNoTearConfiguration ( type, order )
The abstract operation IsNoTearConfiguration takes arguments type (a TypedArray element
type) and order (seq-cst,
unordered, or init) and returns a Boolean. It performs the
following steps when called:
The abstract operation RawBytesToNumeric takes arguments type (a TypedArray element
type), rawBytes (a List of byte values), and
isLittleEndian (a Boolean) and returns a Number or a BigInt. It performs the following steps
when called:
Let elementSize be the Element Size value specified in Table 73 for Element Type
type.
If isLittleEndian is false, reverse the order of the elements of
rawBytes.
If type is float16, then
Let value be the byte elements of rawBytes concatenated and interpreted as
a little-endian bit string encoding of an IEEE 754-2019 binary16 value.
If value is a NaN, return NaN.
Return the Number value that corresponds to value.
If type is float32, then
Let value be the byte elements of rawBytes concatenated and interpreted as
a little-endian bit string encoding of an IEEE 754-2019 binary32 value.
If value is a NaN, return NaN.
Return the Number value that corresponds to value.
If type is float64, then
Let value be the byte elements of rawBytes concatenated and interpreted as
a little-endian bit string encoding of an IEEE 754-2019 binary64 value.
If value is a NaN, return NaN.
Return the Number value that corresponds to value.
Let intValue be the byte elements of rawBytes concatenated and interpreted
as a bit string encoding of an unsigned little-endian binary number.
Else,
Let intValue be the byte elements of rawBytes concatenated and interpreted
as a bit string encoding of a binary little-endian two's complement number of bit length
elementSize × 8.
If IsBigIntElementType(type) is
true, return the BigInt value that corresponds to intValue.
Otherwise, return the Number value that corresponds to intValue.
25.1.3.15 GetRawBytesFromSharedBlock ( block,
byteIndex, type, isTypedArray, order )
The abstract operation GetRawBytesFromSharedBlock takes arguments block (a Shared Data Block),
byteIndex (a non-negative integer), type (a TypedArray element type),
isTypedArray (a Boolean), and order (seq-cst or
unordered) and returns a List of byte values. It
performs the following steps when called:
Let elementSize be the Element Size value specified in Table 73 for Element Type
type.
If isTypedArray is true and IsNoTearConfiguration(type,
order) is true, let noTear be true;
otherwise let noTear be false.
Let rawValue be a List of length
elementSize whose elements are nondeterministically chosen byte values.
NOTE: In implementations, rawValue is the result of a non-atomic or atomic read
instruction on the underlying hardware. The nondeterminism is a semantic prescription of the memory model to
describe observable behaviour of hardware with weak consistency.
Let readEvent be ReadSharedMemory { [[Order]]: order, [[NoTear]]:
noTear, [[Block]]: block, [[ByteIndex]]: byteIndex, [[ElementSize]]:
elementSize }.
Append readEvent to eventsRecord.[[EventList]].
Append Chosen Value Record { [[Event]]: readEvent, [[ChosenValue]]:
rawValue } to execution.[[ChosenValues]].
The abstract operation GetValueFromBuffer takes arguments arrayBuffer (an ArrayBuffer or
SharedArrayBuffer), byteIndex (a non-negative integer), type (a TypedArray element type),
isTypedArray (a Boolean), and order (seq-cst or
unordered) and optional argument isLittleEndian (a Boolean) and returns
a Number or a BigInt. It performs the following steps when called:
Let rawValue be a List whose
elements are bytes from block at indices in the interval from byteIndex
(inclusive) to byteIndex + elementSize (exclusive).
Assert: The number of
elements in rawValue is elementSize.
If isLittleEndian is not present, set isLittleEndian to the value of the [[LittleEndian]] field of the surrounding agent's Agent Record.
The abstract operation NumericToRawBytes takes arguments type (a TypedArray element
type), value (a Number or a BigInt), and isLittleEndian (a
Boolean) and returns a List of byte values. It
performs the following steps when called:
If type is float16, then
Let rawBytes be a List whose
elements are the 2 bytes that are the result of converting value to IEEE
754-2019 binary16 format using roundTiesToEven mode. The bytes are arranged in
little endian order. If value is NaN, rawBytes may be set
to any implementation chosen IEEE 754-2019 binary16 format NaN encoding. An
implementation must always choose the same encoding for each implementation distinguishable
NaN value.
Else if type is float32, then
Let rawBytes be a List whose
elements are the 4 bytes that are the result of converting value to IEEE
754-2019 binary32 format using roundTiesToEven mode. The bytes are arranged in
little endian order. If value is NaN, rawBytes may be set
to any implementation chosen IEEE 754-2019 binary32 format NaN encoding. An
implementation must always choose the same encoding for each implementation distinguishable
NaN value.
Else if type is float64, then
Let rawBytes be a List whose
elements are the 8 bytes that are the IEEE 754-2019 binary64 format encoding of
value. The bytes are arranged in little endian order. If value is
NaN, rawBytes may be set to any implementation chosen IEEE
754-2019 binary64 format NaN encoding. An implementation must always choose the
same encoding for each implementation distinguishable NaN value.
Else,
Let n be the Element Size value specified in Table 73 for Element Type
type.
Let conversionOperation be the abstract operation named in the Conversion Operation
column in Table 73 for Element Type
type.
The abstract operation SetValueInBuffer takes arguments arrayBuffer (an ArrayBuffer or
SharedArrayBuffer), byteIndex (a non-negative integer), type (a TypedArray element type),
value (a Number or a BigInt), isTypedArray (a Boolean), and order
(seq-cst, unordered, or init) and
optional argument isLittleEndian (a Boolean) and returns unused. It
performs the following steps when called:
Store the individual bytes of rawBytes into block, starting at
block[byteIndex].
Return unused.
25.1.3.19 GetModifySetValueInBuffer ( arrayBuffer,
byteIndex, type, value, op )
The abstract operation GetModifySetValueInBuffer takes arguments arrayBuffer (an ArrayBuffer
or a SharedArrayBuffer), byteIndex (a non-negative integer), type (a TypedArray element type),
value (a Number or a BigInt), and op (a read-modify-write modification
function) and returns a Number or a BigInt. It performs the following steps when
called:
Let rawBytesRead be a List of length
elementSize whose elements are nondeterministically chosen byte values.
NOTE: In implementations, rawBytesRead is the result of a load-link, of a
load-exclusive, or of an operand of a read-modify-write instruction on the underlying hardware.
The nondeterminism is a semantic prescription of the memory model to describe observable
behaviour of hardware with weak consistency.
Let rmwEvent be ReadModifyWriteSharedMemory { [[Order]]: seq-cst, [[NoTear]]: true, [[Block]]:
block, [[ByteIndex]]: byteIndex, [[ElementSize]]: elementSize, [[Payload]]: rawBytes, [[ModifyOp]]:
op }.
Append rmwEvent to eventsRecord.[[EventList]].
Append Chosen Value Record { [[Event]]: rmwEvent, [[ChosenValue]]:
rawBytesRead } to execution.[[ChosenValues]].
Else,
Let rawBytesRead be a List of length
elementSize whose elements are the sequence of elementSize bytes starting
with block[byteIndex].
Let rawBytesModified be op(rawBytesRead, rawBytes).
Store the individual bytes of rawBytesModified into block, starting at
block[byteIndex].
is the initial value of the "ArrayBuffer" property of the global object.
creates and initializes a new ArrayBuffer when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to
inherit the specified ArrayBuffer behaviour must include a super call to the ArrayBuffer
constructor to create
and initialize subclass instances with the internal state necessary to support the
ArrayBuffer.prototype built-in methods.
25.1.4.1 ArrayBuffer ( length [ , options ] )
This function performs the following steps when called:
If NewTarget is undefined, throw a TypeError exception.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
25.1.5.3 get ArrayBuffer [ %Symbol.species% ]
ArrayBuffer[%Symbol.species%] is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
Return the this value.
The value of the "name" property of this function is "get
[Symbol.species]".
does not have an [[ArrayBufferData]] or [[ArrayBufferByteLength]] internal slot.
25.1.6.1 get ArrayBuffer.prototype.byteLength
ArrayBuffer.prototype.byteLength is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
The initial value of ArrayBuffer.prototype.constructor is %ArrayBuffer%.
25.1.6.3 get ArrayBuffer.prototype.detached
ArrayBuffer.prototype.detached is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
ArrayBuffer.prototype.maxByteLength is an accessor property whose set accessor function
is undefined. Its get accessor function performs the following steps when called:
ArrayBuffer.prototype.resizable is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
NOTE: Neither creation of the new Data Block nor copying from the old Data Block are
observable. Implementations may implement this method as in-place growth or shrinkage.
Set O.[[ArrayBufferData]] to newBlock.
Set O.[[ArrayBufferByteLength]] to newByteLength.
Return undefined.
25.1.6.7 ArrayBuffer.prototype.slice ( start, end )
This method performs the following steps when called:
The initial value of the %Symbol.toStringTag% property is the String value
"ArrayBuffer".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true }.
25.1.7 Properties of ArrayBuffer Instances
ArrayBuffer instances inherit properties from the ArrayBuffer prototype object.
ArrayBuffer instances each have an [[ArrayBufferData]] internal slot, an [[ArrayBufferByteLength]] internal slot, and an [[ArrayBufferDetachKey]] internal slot. ArrayBuffer instances which are resizable each
have an [[ArrayBufferMaxByteLength]] internal slot.
ArrayBuffer instances whose [[ArrayBufferData]] is null are
considered to be detached and all operators to access or modify data contained in the ArrayBuffer instance
will fail.
ArrayBuffer instances whose [[ArrayBufferDetachKey]] is set to a value other than
undefined need to have all DetachArrayBuffer calls passing that same "detach key" as
an argument, otherwise a TypeError will result. This internal slot is only ever set by certain embedding
environments, not by algorithms in this specification.
25.1.8 Resizable ArrayBuffer Guidelines
Note 1
The following are guidelines for ECMAScript programmers working with resizable ArrayBuffer.
We recommend that programs be tested in their deployment environments where possible. The amount of
available physical memory differs greatly between hardware devices. Similarly, virtual memory subsystems
also differ greatly between hardware devices as well as operating systems. An application that runs
without out-of-memory errors on a 64-bit desktop web browser could run out of memory on a 32-bit mobile
web browser.
When choosing a value for the "maxByteLength" option for resizable ArrayBuffer, we
recommend that the smallest possible size for the application be chosen. We recommend that
"maxByteLength" does not exceed 1,073,741,824 (230 bytes or 1GiB).
Please note that successfully constructing a resizable ArrayBuffer for
a particular maximum size does not guarantee that future resizes will succeed.
Note 2
The following are guidelines for ECMAScript implementers implementing resizable ArrayBuffer.
Resizable ArrayBuffer can
be implemented as copying upon resize, as in-place growth via reserving virtual memory up front, or as a
combination of both for different values of the constructor's "maxByteLength" option.
If a host is multi-tenanted (i.e.
it runs many ECMAScript applications simultaneously), such as a web browser, and its implementations
choose to implement in-place growth by reserving virtual memory, we recommend that both 32-bit and
64-bit implementations throw for values of "maxByteLength" ≥ 1GiB to 1.5GiB. This is
to reduce the likelihood a single application can exhaust the virtual memory address space and to reduce
interoperability risk.
If a host does not have virtual
memory, such as those running on embedded devices without an MMU, or if a host only implements resizing by copying, it may accept
any Number value for the "maxByteLength" option. However,
we recommend a RangeError be thrown if a memory block of the requested size can never
be allocated. For example, if the requested size is greater than the maximum amount of usable memory on
the device.
25.2 SharedArrayBuffer Objects
25.2.1 Fixed-length and Growable SharedArrayBuffer Objects
A fixed-length SharedArrayBuffer is a SharedArrayBuffer whose byte length cannot
change after creation.
The abstract operation AllocateSharedArrayBuffer takes arguments constructor (a constructor) and
byteLength (a non-negative integer) and optional argument maxByteLength (a non-negative
integer or
empty) and returns either a normal completion containing a
SharedArrayBuffer or a throw completion. It is used to create
a SharedArrayBuffer. It performs the following steps when called:
Let slots be « [[ArrayBufferData]] ».
If maxByteLength is present and maxByteLength is not
empty, let allocatingGrowableBuffer be true;
otherwise let allocatingGrowableBuffer be false.
If allocatingGrowableBuffer is true, then
If byteLength > maxByteLength, throw a RangeError
exception.
Append [[ArrayBufferByteLengthData]] and [[ArrayBufferMaxByteLength]] to slots.
Set obj.[[ArrayBufferByteLengthData]] to
byteLengthBlock.
Set obj.[[ArrayBufferMaxByteLength]] to
maxByteLength.
Else,
Set obj.[[ArrayBufferByteLength]] to byteLength.
Return obj.
25.2.2.2 IsSharedArrayBuffer ( obj )
The abstract operation IsSharedArrayBuffer takes argument obj (an ArrayBuffer or a
SharedArrayBuffer) and returns a Boolean. It tests whether an object is an ArrayBuffer, a
SharedArrayBuffer, or a subtype of either. It performs the following steps when called:
The host-defined
abstract operation HostGrowSharedArrayBuffer takes arguments buffer (a SharedArrayBuffer) and
newByteLength (a non-negative integer) and returns either a normal completion containing either
handled or unhandled, or a throw completion. It gives the
host an opportunity to perform
implementation-defined growing of buffer. If
the host chooses not to handle
growing of buffer, it may return unhandled for the default behaviour.
The implementation of HostGrowSharedArrayBuffer must conform to the following requirements:
If the abstract operation does not complete normally with unhandled, and
newByteLength < the current byte length of the buffer or
newByteLength > buffer.[[ArrayBufferMaxByteLength]],
throw a RangeError exception.
Let isLittleEndian be the value of the [[LittleEndian]] field of
the surrounding
agent's Agent
Record. If the abstract operation completes normally with
handled, a WriteSharedMemory or ReadModifyWriteSharedMemory event whose [[Order]] is seq-cst, [[Payload]]
is NumericToRawBytes(biguint64,
newByteLength, isLittleEndian), [[Block]] is
buffer.[[ArrayBufferByteLengthData]], [[ByteIndex]] is 0, and [[ElementSize]] is 8 is added to
the surrounding
agent's candidate execution such that racing calls to
SharedArrayBuffer.prototype.grow are not "lost", i.e. silently do nothing.
Note
The second requirement above is intentionally vague about how or when the current byte length of
buffer is read. Because the byte length must be updated via an atomic read-modify-write
operation on the underlying hardware, architectures that use load-link/store-conditional or
load-exclusive/store-exclusive instruction pairs may wish to keep the paired instructions close in the
instruction stream. As such, SharedArrayBuffer.prototype.grow itself does not perform bounds checking
on newByteLength before calling HostGrowSharedArrayBuffer, nor is there a requirement on
when the current byte length is read.
This is in contrast with HostResizeArrayBuffer, which is guaranteed that
the value of newByteLength is ≥ 0 and ≤ buffer.[[ArrayBufferMaxByteLength]].
The default implementation of HostGrowSharedArrayBuffer is to return NormalCompletion(unhandled).
is the initial value of the "SharedArrayBuffer" property of the global object, if
that property is present (see below).
creates and initializes a new SharedArrayBuffer when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to
inherit the specified SharedArrayBuffer behaviour must include a super call to the
SharedArrayBuffer constructor to create and initialize subclass instances with the
internal state necessary to support the SharedArrayBuffer.prototype built-in methods.
Whenever a host does not provide
concurrent access to SharedArrayBuffers it may omit the "SharedArrayBuffer" property of
the global
object.
Note
Unlike an ArrayBuffer, a SharedArrayBuffer cannot become detached, and its
internal [[ArrayBufferData]] slot is never null.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
25.2.4.2 get SharedArrayBuffer [ %Symbol.species% ]
SharedArrayBuffer[%Symbol.species%] is an accessor property whose set accessor function
is undefined. Its get accessor function performs the following steps when called:
Return the this value.
The value of the "name" property of this function is "get
[Symbol.species]".
25.2.5 Properties of the SharedArrayBuffer Prototype Object
does not have an [[ArrayBufferData]] or [[ArrayBufferByteLength]] internal slot.
25.2.5.1 get SharedArrayBuffer.prototype.byteLength
SharedArrayBuffer.prototype.byteLength is an accessor property whose set accessor function
is undefined. Its get accessor function performs the following steps when called:
Let byteLengthBlock be O.[[ArrayBufferByteLengthData]].
Let currentByteLengthRawBytes be GetRawBytesFromSharedBlock(byteLengthBlock,
0, biguint64, true, seq-cst).
Let newByteLengthRawBytes be NumericToRawBytes(biguint64,
ℤ(newByteLength),
isLittleEndian).
Repeat,
NOTE: This is a compare-and-exchange loop to ensure that parallel, racing grows of the same
buffer are totally ordered, are not lost, and do not silently do nothing. The loop exits if it was
able to attempt to grow uncontended.
Let currentByteLength be ℝ(RawBytesToNumeric(biguint64,
currentByteLengthRawBytes, isLittleEndian)).
If newByteLength = currentByteLength, return undefined.
If newByteLength < currentByteLength or newByteLength >
O.[[ArrayBufferMaxByteLength]], throw a
RangeError exception.
Let byteLengthDelta be newByteLength - currentByteLength.
If it is impossible to create a new Shared Data Block value consisting of
byteLengthDelta bytes, throw a RangeError exception.
NOTE: No new Shared
Data Block is constructed and used here. The observable behaviour of growable
SharedArrayBuffers is specified by allocating a max-sized Shared Data Block at construction time, and this step
captures the requirement that implementations that run out of memory must throw a
RangeError.
Let readByteLengthRawBytes be AtomicCompareExchangeInSharedBlock(byteLengthBlock,
0, 8, currentByteLengthRawBytes, newByteLengthRawBytes).
If ByteListEqual(readByteLengthRawBytes,
currentByteLengthRawBytes) is true, return
undefined.
Set currentByteLengthRawBytes to readByteLengthRawBytes.
Note
Spurious failures of the compare-exchange to update the length are prohibited. If the bounds checking
for the new length passes and the implementation is not out of memory, a ReadModifyWriteSharedMemory event (i.e. a
successful compare-exchange) is always added into the candidate execution.
Parallel calls to SharedArrayBuffer.prototype.grow are totally ordered. For example, consider two
racing calls: sab.grow(10) and sab.grow(20). One of the two calls is
guaranteed to win the race. The call to sab.grow(10) will never shrink sab
even if sab.grow(20) happened first; in that case it will instead throw a RangeError.
25.2.5.4 get SharedArrayBuffer.prototype.growable
SharedArrayBuffer.prototype.growable is an accessor property whose set accessor function
is undefined. Its get accessor function performs the following steps when called:
25.2.5.5 get SharedArrayBuffer.prototype.maxByteLength
SharedArrayBuffer.prototype.maxByteLength is an accessor property whose set accessor function
is undefined. Its get accessor function performs the following steps when called:
The initial value of the %Symbol.toStringTag% property is the String value
"SharedArrayBuffer".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true }.
25.2.6 Properties of SharedArrayBuffer Instances
SharedArrayBuffer instances inherit properties from the SharedArrayBuffer prototype
object. SharedArrayBuffer instances each have an [[ArrayBufferData]] internal slot. SharedArrayBuffer instances which are not growable
each have an [[ArrayBufferByteLength]] internal slot. SharedArrayBuffer instances
which are growable each have an [[ArrayBufferByteLengthData]] internal slot and an
[[ArrayBufferMaxByteLength]] internal slot.
Note
SharedArrayBuffer instances, unlike ArrayBuffer instances, are never detached.
We recommend that programs be tested in their deployment environments where possible. The amount of
available physical memory differ greatly between hardware devices. Similarly, virtual memory subsystems
also differ greatly between hardware devices as well as operating systems. An application that runs
without out-of-memory errors on a 64-bit desktop web browser could run out of memory on a 32-bit mobile
web browser.
When choosing a value for the "maxByteLength" option for growable
SharedArrayBuffer, we recommend that the smallest possible size for the application
be chosen. We recommend that "maxByteLength" does not exceed 1073741824, or 1GiB.
Please note that successfully constructing a growable
SharedArrayBuffer for a particular maximum size does not guarantee that future grows
will succeed.
Not all loads of a growable
SharedArrayBuffer's length are synchronizing seq-cst loads.
Loads of the length that are for bounds-checking of an integer-indexed property access, e.g.
u8[idx], are not synchronizing. In general, in the absence of explicit synchronization, one
property access being in-bound does not imply a subsequent property access in the same agent is also in-bound. In contrast,
explicit loads of the length via the length and byteLength getters on
SharedArrayBuffer, %TypedArray%.prototype, and
DataView.prototype are synchronizing. Loads of the length that are performed by built-in methods to
check if a TypedArray
is entirely out-of-bounds are also synchronizing.
We recommend growable
SharedArrayBuffer be implemented as in-place growth via reserving virtual memory up
front.
Because grow operations can happen in parallel with memory accesses on a growable
SharedArrayBuffer, the constraints of the memory model require that even unordered
accesses do not "tear" (bits of their values will not be mixed). In practice, this means the underlying
data block of a growable
SharedArrayBuffer cannot be grown by being copied without stopping the world. We do
not recommend stopping the world as an implementation strategy because it introduces a serialization
point and is slow.
Grown memory must appear zeroed from the moment of its creation, including to any racy accesses in
parallel. This can be accomplished via zero-filled-on-demand virtual memory pages, or careful
synchronization if manually zeroing memory.
Integer-indexed
property access on TypedArray views of growable SharedArrayBuffers is intended to be
optimizable similarly to access on TypedArray views of non-growable SharedArrayBuffers, because
integer-indexed
property loads on are not synchronizing on the underlying buffer's length (see programmer guidelines
above). For example, bounds checks for property accesses may still be hoisted out of loops.
In practice it is difficult to implement growable
SharedArrayBuffer by copying on hosts that do not have virtual memory, such as those running on embedded
devices without an MMU. Memory usage behaviour of growable SharedArrayBuffers on such hosts may significantly differ from that
of hosts with virtual memory. Such
hosts should clearly communicate
memory usage expectations to users.
25.3 DataView Objects
25.3.1 Abstract Operations For DataView Objects
25.3.1.1 DataView With Buffer Witness Records
A DataView With Buffer Witness
Record is a Record value used to encapsulate a
DataView along with a cached byte length of the viewed buffer. It is used to help ensure there is a single
shared memory read event of the byte length data block when the viewed buffer is a growable
SharedArrayBuffers.
DataView With Buffer Witness Records have the fields listed in Table 75.
The byte length of the object's [[ViewedArrayBuffer]] when the
Record was created.
25.3.1.2 MakeDataViewWithBufferWitnessRecord ( obj,
order )
The abstract operation MakeDataViewWithBufferWitnessRecord takes arguments obj (a DataView)
and order (seq-cst or unordered) and returns a
DataView With Buffer Witness Record.
It performs the following steps when called:
The abstract operation GetViewByteLength takes argument viewRecord (a DataView With Buffer Witness Record)
and returns a non-negative integer. It performs the following steps when called:
The abstract operation IsViewOutOfBounds takes argument viewRecord (a DataView With Buffer Witness Record)
and returns a Boolean. It performs the following steps when called:
Let view be viewRecord.[[Object]].
Let bufferByteLength be viewRecord.[[CachedBufferByteLength]].
Assert: IsDetachedBuffer(view.[[ViewedArrayBuffer]]) is true if and only if
bufferByteLength is detached.
If bufferByteLength is detached, return true.
Let byteOffsetStart be view.[[ByteOffset]].
If view.[[ByteLength]] is auto, then
Let byteOffsetEnd be bufferByteLength.
Else,
Let byteOffsetEnd be byteOffsetStart + view.[[ByteLength]].
If byteOffsetStart > bufferByteLength or byteOffsetEnd >
bufferByteLength, return true.
NOTE: 0-length DataViews are not considered out-of-bounds.
Return false.
25.3.1.5 GetViewValue ( view, requestIndex,
isLittleEndian, type )
is the initial value of the "DataView" property of the global object.
creates and initializes a new DataView when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to
inherit the specified DataView behaviour must include a super call to the DataView constructor to create and
initialize subclass instances with the internal state necessary to support the
DataView.prototype built-in methods.
does not have a [[DataView]], [[ViewedArrayBuffer]],
[[ByteLength]], or [[ByteOffset]] internal slot.
25.3.4.1 get DataView.prototype.buffer
DataView.prototype.buffer is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
Assert: O has a
[[ViewedArrayBuffer]] internal slot.
Let buffer be O.[[ViewedArrayBuffer]].
Return buffer.
25.3.4.2 get DataView.prototype.byteLength
DataView.prototype.byteLength is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
DataView.prototype.byteOffset is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
The initial value of the %Symbol.toStringTag% property is the String value
"DataView".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true }.
25.3.5 Properties of DataView Instances
DataView instances are ordinary objects that inherit properties from the DataView prototype object.
DataView instances each have [[DataView]], [[ViewedArrayBuffer]], [[ByteLength]], and [[ByteOffset]] internal slots.
Note
The value of the [[DataView]] internal slot is not used within this
specification. The simple presence of that internal slot is used within the specification to identify
objects created using the DataView constructor.
25.4 The Atomics Object
The Atomics object:
is %Atomics%.
is the initial value of the "Atomics" property of the global object.
does not have a [[Construct]] internal method; it cannot be used as a constructor with the
new operator.
does not have a [[Call]] internal method; it cannot be invoked as a function.
The Atomics object provides functions that operate indivisibly (atomically) on shared memory array cells as
well as functions that let agents wait
for and dispatch primitive events. When used with discipline, the Atomics functions allow multi-agent programs that communicate through shared
memory to execute in a well-understood order even on parallel CPUs. The rules that govern shared-memory
communication are provided by the memory model, defined below.
Note
For informative guidelines for programming and implementing shared memory in ECMAScript, please see the
notes at the end of the memory model section.
25.4.1 Waiter Record
A Waiter Record is a Record value used to denote a particular
call to Atomics.wait or Atomics.waitAsync.
The agent
cluster has a store of WaiterList Records; the store is indexed by (block,
i), where block is a Shared Data Block and i a byte offset into the memory
of block. WaiterList Records are agent-independent: a lookup in the store of WaiterList Records by
(block, i) will result in the same WaiterList Record in any agent in the agent cluster.
Each WaiterList Record has a critical section that
controls exclusive access to that WaiterList Record during evaluation. Only a single agent may enter a WaiterList Record's critical section at
one time. Entering and leaving a WaiterList Record's critical section is controlled by the abstract operationsEnterCriticalSection and LeaveCriticalSection. Operations on a WaiterList
Record—adding and removing waiting agents, traversing the list of agents, suspending and notifying agents on the list, setting and retrieving the Synchronize
event—may only be performed by agents that have entered the WaiterList Record's critical section.
The abstract operation RevalidateAtomicAccess takes arguments typedArray (a TypedArray) and
byteIndexInBuffer (an integer) and returns either a normal completion containingunused or a throw completion.
This operation revalidates the index within the backing buffer for atomic operations after all argument
coercions are performed in Atomics methods, as argument coercions can have arbitrary side effects, which
could cause the buffer to become out of bounds. This operation does not throw when typedArray's
backing buffer is a SharedArrayBuffer. It performs the following steps when called:
If byteIndexInBuffer ≥ taRecord.[[CachedBufferByteLength]], throw a RangeError exception.
Return unused.
25.4.3.5 GetWaiterList ( block, i )
The abstract operation GetWaiterList takes arguments block (a Shared Data Block) and
i (a non-negative integer that is evenly divisible by 4) and returns a WaiterList
Record. It performs the following steps when called:
Assert: i and
i + 3 are valid byte offsets within the memory of block.
Return the WaiterList Record that is referenced by the pair
(block, i).
25.4.3.6 EnterCriticalSection ( WL )
The abstract operation EnterCriticalSection takes argument WL (a WaiterList
Record) and returns unused. It performs the following steps when
called:
Append (WL.[[MostRecentLeaveEvent]], enterEvent)
to eventsRecord.[[AgentSynchronizesWith]].
Return unused.
EnterCriticalSection has contention when an agent attempting to enter the critical
section must wait for another agent to leave it. When there is no contention, FIFO order of
EnterCriticalSection calls is observable. When there is contention, an implementation may choose an
arbitrary order but may not cause an agent to wait indefinitely.
25.4.3.7 LeaveCriticalSection ( WL )
The abstract operation LeaveCriticalSection takes argument WL (a WaiterList
Record) and returns unused. It performs the following steps when
called:
The abstract operation AddWaiter takes arguments WL (a WaiterList
Record) and waiterRecord (a Waiter Record) and returns
unused. It performs the following steps when called:
Assert: There is no
Waiter
Record in WL.[[Waiters]] whose [[PromiseCapability]] field is waiterRecord.[[PromiseCapability]] and whose [[AgentSignifier]]
field is waiterRecord.[[AgentSignifier]].
Append waiterRecord to WL.[[Waiters]].
Return unused.
25.4.3.9 RemoveWaiter ( WL, waiterRecord )
The abstract operation RemoveWaiter takes arguments WL (a WaiterList
Record) and waiterRecord (a Waiter Record) and returns
unused. It performs the following steps when called:
The abstract operation RemoveWaiters takes arguments WL (a WaiterList
Record) and c (a non-negative integer or +∞) and returns a List of Waiter Records. It
performs the following steps when called:
Let L be a List whose elements are the first
n elements of WL.[[Waiters]].
Remove the first n elements of WL.[[Waiters]].
Return L.
25.4.3.11 SuspendThisAgent ( WL, waiterRecord )
The abstract operation SuspendThisAgent takes arguments WL (a WaiterList
Record) and waiterRecord (a Waiter Record) and returns
unused. It performs the following steps when called:
Perform LeaveCriticalSection(WL) and suspend
the surrounding
agent until the time is waiterRecord.[[TimeoutTime]], performing the combined operation in such a way that a
notification that arrives after the critical section is exited but before the suspension
takes effect is not lost. The surrounding agent can only wake from suspension due to a
timeout or due to another agent calling NotifyWaiter with arguments WL and
thisAgent (i.e. via a call to Atomics.notify).
The abstract operation NotifyWaiter takes arguments WL (a WaiterList
Record) and waiterRecord (a Waiter Record) and returns
unused. It performs the following steps when called:
The abstract operation EnqueueResolveInAgentJob takes arguments agentSignifier (an agent signifier),
promiseCapability (a PromiseCapability Record), and
resolution ("ok" or "timed-out") and returns
unused. It performs the following steps when called:
Let resolveJob be a new JobAbstract Closure with no parameters that captures
agentSignifier, promiseCapability, and resolution and performs the
following steps when called:
NOTE: There is no special handling of synchronous immediate timeouts. Asynchronous immediate
timeouts have special handling in order to fail fast and avoid unnecessary Promise jobs.
Let timeoutTime be ℝ(now) + t + additionalTimeout.
NOTE: When t is +∞, timeoutTime is also +∞.
Let waiterRecord be a new Waiter Record { [[AgentSignifier]]: thisAgent, [[PromiseCapability]]: promiseCapability, [[TimeoutTime]]: timeoutTime, [[Result]]:
"ok" }.
additionalTimeout allows implementations to pad timeouts as necessary, such as for
reducing power consumption or coarsening timer resolution to mitigate timing attacks. This value may
differ from call to call of DoWait.
The abstract operation EnqueueAtomicsWaitAsyncTimeoutJob takes arguments WL (a WaiterList
Record) and waiterRecord (a Waiter Record) and returns
unused. It performs the following steps when called:
Let timeoutJob be a new JobAbstract Closure with no parameters that captures
WL and waiterRecord and performs the following steps when called:
The abstract operation AtomicCompareExchangeInSharedBlock takes arguments block (a Shared Data Block),
byteIndexInBuffer (an integer), elementSize (a non-negative integer), expectedBytes (a
List of byte values), and
replacementBytes (a List of byte values) and
returns a List of byte values. It
performs the following steps when called:
Let rawBytesRead be a List of length
elementSize whose elements are nondeterministically chosen byte values.
NOTE: In implementations, rawBytesRead is the result of a load-link, of a load-exclusive,
or of an operand of a read-modify-write instruction on the underlying hardware. The nondeterminism is
a semantic prescription of the memory model to describe observable behaviour of hardware
with weak consistency.
NOTE: The comparison of the expected value and the read value is performed outside of the read-modify-write modification function to avoid
needlessly strong synchronization when the expected value is not equal to the read value.
If ByteListEqual(rawBytesRead,
expectedBytes) is true, then
Let second be a new read-modify-write modification function with
parameters (oldBytes, newBytes) that captures nothing and performs the
following steps atomically when called:
Return newBytes.
Let event be ReadModifyWriteSharedMemory { [[Order]]: seq-cst, [[NoTear]]: true, [[Block]]:
block, [[ByteIndex]]: byteIndexInBuffer, [[ElementSize]]: elementSize, [[Payload]]: replacementBytes, [[ModifyOp]]: second }.
Else,
Let event be ReadSharedMemory { [[Order]]: seq-cst, [[NoTear]]: true, [[Block]]:
block, [[ByteIndex]]: byteIndexInBuffer, [[ElementSize]]: elementSize }.
Append event to eventsRecord.[[EventList]].
Append Chosen Value Record { [[Event]]: event, [[ChosenValue]]:
rawBytesRead } to execution.[[ChosenValues]].
Return rawBytesRead.
25.4.3.17 AtomicReadModifyWrite ( typedArray, index,
value, op )
The abstract operation ByteListBitwiseOp takes arguments op (&,
^, or |), xBytes (a List of byte values), and
yBytes (a List of byte values) and
returns a List of byte values. The
operation atomically performs a bitwise operation on all byte values of the arguments and returns a
List of byte values. It
performs the following steps when called:
Assert: xBytes
and yBytes have the same number of elements.
Let resultByte be the result of applying the bitwise inclusive OR operation to
xByte and yByte.
Set i to i + 1.
Append resultByte to result.
Return result.
25.4.3.19 ByteListEqual ( xBytes, yBytes )
The abstract operation ByteListEqual takes arguments xBytes (a List of byte values) and
yBytes (a List of byte values) and
returns a Boolean. It performs the following steps when called:
If xBytes and yBytes do not have the same number of elements, return
false.
Let i be 0.
For each element xByte of xBytes, do
Let yByte be yBytes[i].
If xByte ≠ yByte, return false.
Set i to i + 1.
Return true.
25.4.4 Atomics.add ( typedArray, index, value
)
This function performs the following steps when called:
Let add be a new read-modify-write modification function with
parameters (xBytes, yBytes) that captures typedArray and performs the
following steps atomically when called:
This function performs the following steps when called:
Let and be a new read-modify-write modification function with
parameters (xBytes, yBytes) that captures nothing and performs the following steps
atomically when called:
25.4.7 Atomics.exchange ( typedArray, index,
value )
This function performs the following steps when called:
Let second be a new read-modify-write modification function with
parameters (oldBytes, newBytes) that captures nothing and performs the following
steps atomically when called:
This function is an optimization primitive. The intuition is that if the atomic step of an atomic
primitive (compareExchange, load, store, add,
sub, and, or, xor, or exchange) on a
datum of size n bytes will be performed without the surrounding agent acquiring a lock outside
the n bytes comprising the datum, then Atomics.isLockFree(n) will
return true. High-performance algorithms will use this function to determine whether
to use locks or atomic operations in critical sections. If an atomic primitive is not
lock-free then it is often more efficient for an algorithm to provide its own locking.
Atomics.isLockFree(4) always returns true as that can be supported on
all known relevant hardware. Being able to assume this will generally simplify programs.
Regardless of the value returned by this function, all atomic operations are guaranteed to be atomic.
For example, they will never have a visible operation take place in the middle of the operation (e.g.,
"tearing").
25.4.9 Atomics.load ( typedArray, index )
This function performs the following steps when called:
This function performs the following steps when called:
Let or be a new read-modify-write modification function with
parameters (xBytes, yBytes) that captures nothing and performs the following steps
atomically when called:
This function performs the following steps when called:
Let subtract be a new read-modify-write modification function with
parameters (xBytes, yBytes) that captures typedArray and performs the
following steps atomically when called:
This function puts the surrounding agent in a wait queue and suspends it until
notified or until the wait times out, returning a String differentiating those cases.
This function performs the following steps when called:
Let xor be a new read-modify-write modification function with
parameters (xBytes, yBytes) that captures nothing and performs the following steps
atomically when called:
does not have a [[Construct]] internal method; it cannot be used as a constructor with the
new operator.
does not have a [[Call]] internal method; it cannot be invoked as a function.
The JSON Data Interchange Format is defined in ECMA-404. The JSON interchange format used in this
specification is exactly that described by ECMA-404. Conforming implementations of JSON.parse and
JSON.stringify must support the exact interchange format described in the ECMA-404 specification
without any deletions or extensions to the format.
25.5.1 JSON.parse ( text [ , reviver ] )
This function parses a JSON text (a JSON-formatted String) and produces an ECMAScript
language value. The JSON format represents literals, arrays, and objects with a syntax
similar to the syntax for ECMAScript literals, Array Initializers, and Object Initializers. After parsing,
JSON objects are realized as ECMAScript objects. JSON arrays are realized as ECMAScript Array instances.
JSON strings, numbers, booleans, and null are realized as ECMAScript Strings, Numbers, Booleans, and
null.
The optional reviver parameter is a function that takes two parameters, key and
value. It can filter and transform the results. It is called with each of the
key/value pairs produced by the parse, and its return value is used instead of the
original value. If it returns what it received, the structure is not modified. If it returns
undefined then the property is deleted from the result.
Assert: result is either a String, a Number, a Boolean,
an Object that is defined by either an ArrayLiteral or an ObjectLiteral, or null.
Return result.
It is not permitted for a conforming implementation of JSON.parse to extend the JSON
grammars. If an implementation wishes to support a modified or extended JSON interchange format it must do
so by defining a different parse function.
Note 1
Valid JSON text is a subset of the ECMAScript PrimaryExpression syntax. Step 1
verifies that jsonString conforms to that subset, and step 8 asserts that evaluation returns a value of an
appropriate type.
However, because 13.2.5.5
behaves differently during ParseJSON, the same source text can produce different results when
evaluated as a PrimaryExpression rather than as JSON. Furthermore,
the Early Error for duplicate "__proto__" properties in object literals, which
likewise does not apply during ParseJSON, means that not all texts accepted by ParseJSON are valid as
a PrimaryExpression, despite
matching the grammar.
Note 2
In the case where there are duplicate name Strings within an object, lexically preceding values for
the same key shall be overwritten.
25.5.2 JSON.stringify ( value [ , replacer [ ,
space ] ] )
This function returns a String in UTF-16 encoded JSON format representing an ECMAScript
language value, or undefined. It can take three parameters. The
value parameter is an ECMAScript language value, which is usually an
object or array, although it can also be a String, Boolean, Number or null. The optional
replacer parameter is either a function that alters the way objects and arrays are stringified,
or an array of Strings and Numbers that acts as an inclusion list for selecting the object properties that
will be stringified. The optional space parameter is a String or Number that allows the
result to have white space injected into it to improve human readability.
Let state be the JSON Serialization Record { [[ReplacerFunction]]: ReplacerFunction, [[Stack]]: stack, [[Indent]]:
indent, [[Gap]]: gap, [[PropertyList]]: PropertyList }.
JSON structures are allowed to be nested to any depth, but they must be acyclic. If value is
or contains a cyclic structure, then this function must throw a TypeError exception.
This is an example of a value that cannot be stringified:
a = [];
a[0] = a;
my_text = JSON.stringify(a); // This must throw a TypeError.
Note 2
Symbolic primitive values are rendered as follows:
The null value is rendered in JSON text as the String value
"null".
The undefined value is not rendered.
The true value is rendered in JSON text as the String value
"true".
The false value is rendered in JSON text as the String value
"false".
Note 3
String values are wrapped in QUOTATION MARK (") code units. The code units "
and \ are escaped with \ prefixes. Control characters code units are replaced
with escape sequences \uHHHH, or with the shorter forms, \b (BACKSPACE),
\f (FORM FEED), \n (LINE FEED), \r (CARRIAGE RETURN),
\t (CHARACTER TABULATION).
Note 4
Finite numbers are
stringified as if by calling ToString(number). NaN and
Infinity regardless of sign are represented as the String value
"null".
Note 5
Values that do not have a JSON representation (such as undefined and functions) do
not produce a String. Instead they produce the undefined value. In arrays these
values are represented as the String value "null". In objects an unrepresentable
value causes the property to be excluded from stringification.
Note 6
An object is rendered as U+007B (LEFT CURLY BRACKET) followed by zero or more properties, separated
with a U+002C (COMMA), closed with a U+007D (RIGHT CURLY BRACKET). A property is a quoted String
representing the property
name, a U+003A (COLON), and then the stringified property value. An array is rendered
as an opening U+005B (LEFT SQUARE BRACKET) followed by zero or more values, separated with a U+002C
(COMMA), closed with a U+005D (RIGHT SQUARE BRACKET).
25.5.2.1 JSON Serialization Record
A JSON Serialization Record is a Record value used to enable
serialization to the JSON format.
JSON Serialization Records have the fields listed in Table 78.
The abstract operation SerializeJSONProperty takes arguments state (a JSON
Serialization Record), key (a String), and holder (an Object) and
returns either a normal completion containing either a
String or undefined, or a throw completion. It
performs the following steps when called:
The abstract operation QuoteJSONString takes argument value (a String) and returns a String.
It wraps value in 0x0022 (QUOTATION MARK) code units and escapes certain other code units
within it. This operation interprets value as a sequence of UTF-16 encoded code points, as
described in 6.1.4. It performs the following
steps when called:
Let product be the String value consisting solely of the code unit 0x0022 (QUOTATION
MARK).
If C is listed in the “Code Point” column of Table 79, then
Set product to the string-concatenation of product and
the escape sequence for C as specified in the “Escape Sequence” column of the
corresponding row.
Set product to the string-concatenation of product and the
code unit 0x0022 (QUOTATION MARK).
Return product.
Table 79: JSON Single Character Escape Sequences
Code Point
Unicode Character Name
Escape Sequence
U+0008
BACKSPACE
\b
U+0009
CHARACTER TABULATION
\t
U+000A
LINE FEED (LF)
\n
U+000C
FORM FEED (FF)
\f
U+000D
CARRIAGE RETURN (CR)
\r
U+0022
QUOTATION MARK
\"
U+005C
REVERSE SOLIDUS
\\
25.5.2.4 UnicodeEscape ( C )
The abstract operation UnicodeEscape takes argument C (a code unit) and returns a String. It
represents C as a Unicode escape sequence. It performs the following steps when called:
Let properties be the String value formed by concatenating all the element
Strings of partial with each adjacent pair of Strings separated with the code unit
0x002C (COMMA). A comma is not inserted either before the first String or after the last
String.
Let separator be the string-concatenation of the code unit 0x002C
(COMMA), the code unit 0x000A (LINE FEED), and state.[[Indent]].
Let properties be the String value formed by concatenating all the element
Strings of partial with each adjacent pair of Strings separated with
separator. The separator String is not inserted either before the first
String or after the last String.
Let final be the string-concatenation of
"{", the code unit 0x000A (LINE FEED), state.[[Indent]], properties, the code unit 0x000A (LINE FEED),
stepBack, and "}".
Let properties be the String value formed by concatenating all the element
Strings of partial with each adjacent pair of Strings separated with the code unit
0x002C (COMMA). A comma is not inserted either before the first String or after the last
String.
Let separator be the string-concatenation of the code unit 0x002C
(COMMA), the code unit 0x000A (LINE FEED), and state.[[Indent]].
Let properties be the String value formed by concatenating all the element
Strings of partial with each adjacent pair of Strings separated with
separator. The separator String is not inserted either before the first
String or after the last String.
Let final be the string-concatenation of
"[", the code unit 0x000A (LINE FEED), state.[[Indent]], properties, the code unit 0x000A (LINE FEED),
stepBack, and "]".
Remove the last element of state.[[Stack]].
Set state.[[Indent]] to stepBack.
Return final.
Note
The representation of arrays includes only the elements in the interval from +0𝔽
(inclusive) to array.length (exclusive). Properties whose keys are not array indices are excluded
from the stringification. An array is stringified as an opening LEFT SQUARE BRACKET, elements
separated by COMMA, and a closing RIGHT SQUARE BRACKET.
25.5.3 JSON [ %Symbol.toStringTag% ]
The initial value of the %Symbol.toStringTag% property is the String value
"JSON".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true }.
26 Managing Memory
26.1 WeakRef Objects
A WeakRef is an object that is used to refer to a target
object or symbol without preserving it from garbage collection. WeakRefs can be dereferenced to allow
access to the target value, if the target hasn't been reclaimed by garbage collection.
is the initial value of the "WeakRef" property of the global object.
creates and initializes a new WeakRef when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value in an extends clause of a class definition. Subclass constructors that intend to
inherit the specified WeakRef behaviour must include a super call to the
WeakRefconstructor to create and initialize the subclass instance with the
internal state necessary to support the WeakRef.prototype built-in methods.
26.1.1.1 WeakRef ( target )
This function performs the following steps when called:
If NewTarget is undefined, throw a TypeError exception.
If CanBeHeldWeakly(target) is
false, throw a TypeError exception.
If the WeakRef returns a target value that is
not undefined, then this target value should not be garbage collected
until the current execution of ECMAScript code has completed. The AddToKeptObjects operation makes sure
read consistency is maintained.
let target = { foo() {} };
let weakRef = newWeakRef(target);
// ... later ...if (weakRef.deref()) {
weakRef.deref().foo();
}
In the above example, if the first deref does not evaluate to undefined then the
second deref cannot either.
The initial value of the %Symbol.toStringTag% property is the String value
"WeakRef".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true }.
26.1.4 WeakRef Abstract Operations
26.1.4.1 WeakRefDeref ( weakRef )
The abstract operation WeakRefDeref takes argument weakRef (a WeakRef) and returns an ECMAScript language value. It performs the
following steps when called:
A FinalizationRegistry is an object that
manages registration and unregistration of cleanup operations that are performed when target objects and
symbols are garbage collected.
is the initial value of the "FinalizationRegistry" property of the global object.
creates and initializes a new FinalizationRegistry when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value in an extends clause of a class definition. Subclass constructors that intend to
inherit the specified FinalizationRegistry behaviour must include a super call
to the FinalizationRegistryconstructor to create and initialize the subclass instance with the
internal state necessary to support the FinalizationRegistry.prototype built-in methods.
26.2.1.1 FinalizationRegistry ( cleanupCallback )
This function performs the following steps when called:
If NewTarget is undefined, throw a TypeError exception.
If IsCallable(cleanupCallback) is
false, throw a TypeError exception.
Let finalizationRegistry be ? OrdinaryCreateFromConstructor(NewTarget,
"%FinalizationRegistry.prototype%", « [[Realm]], [[CleanupCallback]], [[Cells]] »).
If unregisterToken is not undefined, throw a
TypeError exception.
Set unregisterToken to empty.
Let cell be the Record { [[WeakRefTarget]]: target, [[HeldValue]]:
heldValue, [[UnregisterToken]]: unregisterToken }.
Append cell to finalizationRegistry.[[Cells]].
Return undefined.
Note
Based on the algorithms and definitions in this specification, cell.[[HeldValue]] is live when finalizationRegistry.[[Cells]] contains cell; however, this does not necessarily mean that
cell.[[UnregisterToken]] or cell.[[Target]] are live. For example, registering an object with itself as its
unregister token would not keep the object alive forever.
An interface is a set of property
keys whose associated values match a specific specification. Any object that provides all
the properties as described by an interface's specification conforms to that interface. An
interface is not represented by a distinct object. There may be many separately implemented objects that
conform to any interface. An individual object may conform to multiple interfaces.
27.1.1.1 The Iterable Interface
The iterable
interface includes the property described in Table 80:
The returned object must conform to the IteratorResult
interface. If a previous call to the next method of an iterator has returned an IteratorResult object whose
"done" property is true, then all subsequent calls to the
next method of that object should also return an IteratorResult object whose
"done" property is true. However, this requirement is not
enforced.
Note 1
Arguments may be passed to the next function but their interpretation and validity is
dependent upon the target iterator. The for-of statement and other common users of iterators do not
pass any arguments, so iterator objects that expect to be used in such a manner must be prepared to
deal with being called with no arguments.
The returned object must conform to the IteratorResult
interface. Invoking this method notifies the iterator
object that the caller does not intend to make any more next
method calls to the iterator. The returned IteratorResult object will typically
have a "done" property whose value is true, and a
"value" property with the value passed as the argument of the
return method. However, this requirement is not enforced.
The returned object must conform to the IteratorResult
interface. Invoking this method notifies the iterator
object that the caller has detected an error condition. The argument may be
used to identify the error condition and typically will be an exception object. A typical
response is to throw the value passed as the argument. If the method does not
throw, the returned IteratorResult object
will typically have a "done" property whose value is true.
Note 2
Typically callers of these methods should check for their existence before invoking them. Certain
ECMAScript language features including for-of, yield*, and
array destructuring call these methods after performing an existence check. Most ECMAScript library
functions that accept iterable objects as arguments also conditionally
call them.
27.1.1.3 The Async Iterable Interface
The async iterable interface includes the properties described in Table 83:
An object that implements the async iterator interface must include the properties in Table
84. Such objects may also implement the properties in Table
85.
The returned promise, when fulfilled, must fulfill with an object that conforms to the
IteratorResult interface. If a
previous call to the next method of an async iterator has returned a promise
for an IteratorResult object whose
"done" property is true, then all subsequent calls to
the next method of that object should also return a promise for an IteratorResult object whose
"done" property is true. However, this requirement is
not enforced.
Additionally, the IteratorResult object that serves as a
fulfillment value should have a "value" property whose value is not a
promise (or "thenable"). However, this requirement is also not enforced.
Note 1
Arguments may be passed to the next function but their interpretation and validity is
dependent upon the target async iterator. The for-await-of
statement and other common users of async iterators do not pass any arguments, so async iterator
objects that expect to be used in such a manner must be prepared to deal with being called with no
arguments.
The returned promise, when fulfilled, must fulfill with an object that conforms to the
IteratorResult interface. Invoking
this method notifies the async iterator object that the caller
does not intend to make any more next method calls to the async iterator. The returned promise
will fulfill with an IteratorResult object which will
typically have a "done" property whose value is true,
and a "value" property with the value passed as the argument of the
return method. However, this requirement is not enforced.
Additionally, the IteratorResult object that serves as a
fulfillment value should have a "value" property whose value is not a
promise (or "thenable"). If the argument value is used in the typical manner, then if it is a
rejected promise, a promise rejected with the same reason should be returned; if it is a
fulfilled promise, then its fulfillment value should be used as the "value"
property of the returned promise's IteratorResult object
fulfillment value. However, these requirements are also not enforced.
The returned promise, when fulfilled, must fulfill with an object that conforms to the
IteratorResult interface. Invoking
this method notifies the async iterator object that the caller
has detected an error condition. The argument may be used to identify the error condition and
typically will be an exception object. A typical response is to return a rejected promise
which rejects with the value passed as the argument.
If the returned promise is fulfilled, the IteratorResult object
fulfillment value will typically have a "done" property whose value is
true. Additionally, it should have a "value" property
whose value is not a promise (or "thenable"), but this requirement is not enforced.
Note 2
Typically callers of these methods should check for their existence before invoking them. Certain
ECMAScript language features including for-await-of and
yield* call these methods after performing an existence check.
27.1.1.5 The IteratorResult Interface
The IteratorResult
interface includes the properties listed in Table 86:
Table 86: IteratorResult Interface Properties
Property
Value
Requirements
"done"
a Boolean
This is the result status of an iteratornext method call. If the
end of the iterator was reached "done"
is true. If the end was not reached "done" is
false and a value is available. If a "done" property
(either own or inherited) does not exist, it is considered to have the value
false.
If done is false, this is the current iteration element value. If done is
true, this is the return value of the iterator, if it supplied one. If the iterator does not have a return value,
"value" is undefined. In that case, the
"value" property may be absent from the conforming object if it does not
inherit an explicit "value" property.
27.1.2 Iterator Helper Objects
An Iterator Helper object is an ordinary object that
represents a lazy transformation of some specific source iterator object. There is not a named
constructor for Iterator
Helper objects. Instead, Iterator Helper objects are created by calling certain methods of Iterator
instance objects.
NOTE: Once a generator enters the completed state it never leaves it and its associated
execution context is never resumed. Any
execution state associated with O can be discarded at this point.
All objects defined in this specification that implement the iterator interface also inherit from
%Iterator.prototype%. ECMAScript code may also define objects that inherit from %Iterator.prototype%.
%Iterator.prototype% provides a place where additional methods that are applicable to all iterator
objects may be added.
The following expression is one way that ECMAScript code can access the %Iterator.prototype% object:
Iterator.prototype.constructor is an accessor property with attributes { [[Enumerable]]: false, [[Configurable]]:
true }. The [[Get]] and [[Set]]
attributes are defined as follows:
27.1.4.1.1 get Iterator.prototype.constructor
The value of the [[Get]] attribute is a built-in function that requires no
arguments. It performs the following steps when called:
Iterator.prototype[%Symbol.toStringTag%] is an accessor property with attributes { [[Enumerable]]: false, [[Configurable]]:
true }. The [[Get]] and [[Set]]
attributes are defined as follows:
27.1.4.14.1 get Iterator.prototype [ %Symbol.toStringTag% ]
The value of the [[Get]] attribute is a built-in function that requires no
arguments. It performs the following steps when called:
Return "Iterator".
27.1.4.14.2 set Iterator.prototype [ %Symbol.toStringTag% ]
The value of the [[Set]] attribute is a built-in function that takes an
argument v. It performs the following steps when called:
All objects defined in this specification that implement the async
iterator interface also inherit from %AsyncIteratorPrototype%. ECMAScript code may
also define objects that inherit from %AsyncIteratorPrototype%. The %AsyncIteratorPrototype% object
provides a place where additional methods that are applicable to all async
iterator objects may be added.
This function performs the following steps when called:
Return the this value.
The value of the "name" property of this function is
"[Symbol.asyncIterator]".
27.1.6 Async-from-Sync Iterator Objects
An Async-from-Sync Iterator object is
an async iterator that adapts a specific synchronous
iterator. Async-from-Sync Iterator objects are never
directly accessible to ECMAScript code. There is not a named constructor for Async-from-Sync Iterator objects.
Instead, Async-from-Sync Iterator objects are created by the CreateAsyncFromSyncIterator
abstract operation as needed.
The abstract operation CreateAsyncFromSyncIterator takes argument syncIteratorRecord (an
Iterator
Record) and returns an Iterator Record. It is used to create an async Iterator
Record from a synchronous Iterator Record. It performs the following steps when
called:
The abstract operation AsyncFromSyncIteratorContinuation takes arguments result (an Object),
promiseCapability (a PromiseCapability Record for an intrinsic
%Promise%), syncIteratorRecord (an Iterator
Record), and closeOnRejection (a Boolean) and returns a Promise. It performs
the following steps when called:
NOTE: Because promiseCapability is derived from the intrinsic %Promise%, the calls to
promiseCapability.[[Reject]] entailed by the use IfAbruptRejectPromise below are guaranteed not to
throw.
NOTE: onFulfilled is used when processing the "value" property of an
IteratorResult object in order to wait for its
value if it is a promise and re-package the result in a new "unwrapped" IteratorResult object.
If done is true, or if closeOnRejection is
false, then
Let onRejected be undefined.
Else,
Let closeIterator be a new Abstract Closure with parameters
(error) that captures syncIteratorRecord and performs the following steps
when called:
A Promise is an object that is used as a placeholder for the eventual results of a deferred (and possibly
asynchronous) computation.
Any Promise is in one of three mutually exclusive states: fulfilled, rejected, and
pending:
A promise p is fulfilled if p.then(f, r) will immediately enqueue a Job to call the function f.
A promise p is rejected if p.then(f, r) will immediately enqueue a Job to call the function r.
A promise is pending if it is neither fulfilled nor rejected.
A promise is said to be settled if it is not pending, i.e. if it is either fulfilled or rejected.
A promise is resolved if it is settled or if it has been “locked in” to match the state of another
promise. Attempting to resolve or reject a resolved promise has no effect. A promise is unresolved if
it is not resolved. An unresolved promise is always in the pending state. A resolved promise may be pending,
fulfilled or rejected.
27.2.1 Promise Abstract Operations
27.2.1.1 PromiseCapability Records
A PromiseCapability Record is a Record value used to encapsulate a
Promise or promise-like object along with the functions that are capable of resolving or rejecting that
promise. PromiseCapability Records are produced by the NewPromiseCapability abstract
operation.
PromiseCapability Records have the fields listed in Table 88.
A PromiseReaction Record is a Record value used to store information
about how a promise should react when it becomes resolved or rejected with a given value. PromiseReaction
Records are created by the PerformPromiseThen abstract operation, and are used by
the Abstract
Closure returned by NewPromiseReactionJob.
PromiseReaction Records have the fields listed in Table 89.
The function that should be applied to the incoming value, and whose return value will govern
what happens to the derived promise. If [[Handler]] is
empty, a function that depends on the value of [[Type]] will be used instead.
27.2.1.3 CreateResolvingFunctions ( promise )
The abstract operation CreateResolvingFunctions takes argument promise (a Promise) and returns
a Record with fields [[Resolve]] (a function object) and [[Reject]] (a
function
object). It performs the following steps when called:
Let alreadyResolved be the Record { [[Value]]: false }.
The "length" property of a promise resolve function is
1𝔽.
27.2.1.4 FulfillPromise ( promise, value )
The abstract operation FulfillPromise takes arguments promise (a Promise) and value
(an ECMAScript language value) and returns
unused. It performs the following steps when called:
If IsConstructor(C) is false,
throw a TypeError exception.
NOTE: C is assumed to be a constructor function that supports the parameter conventions of
the Promise constructor (see 27.2.3.1).
Let resolvingFunctions be the Record { [[Resolve]]: undefined, [[Reject]]:
undefined }.
Let executorClosure be a new Abstract Closure with parameters (resolve,
reject) that captures resolvingFunctions and performs the following steps when
called:
If resolvingFunctions.[[Resolve]] is not
undefined, throw a TypeError exception.
If resolvingFunctions.[[Reject]] is not
undefined, throw a TypeError exception.
If IsCallable(resolvingFunctions.[[Resolve]]) is false, throw a TypeError
exception.
If IsCallable(resolvingFunctions.[[Reject]]) is false, throw a TypeError
exception.
Return the PromiseCapability Record { [[Promise]]: promise, [[Resolve]]:
resolvingFunctions.[[Resolve]], [[Reject]]: resolvingFunctions.[[Reject]] }.
Note
This abstract operation supports Promise subclassing, as it is generic on any constructor that calls a
passed executor function argument in the same way as the Promise constructor. It is used to generalize static
methods of the Promise constructor to any subclass.
27.2.1.6 IsPromise ( x )
The abstract operation IsPromise takes argument x (an ECMAScript language value) and returns a Boolean.
It checks for the promise brand on an object. It performs the following steps when called:
If x does not have a [[PromiseState]] internal slot, return
false.
Return true.
27.2.1.7 RejectPromise ( promise, reason )
The abstract operation RejectPromise takes arguments promise (a Promise) and reason
(an ECMAScript language value) and returns
unused. It performs the following steps when called:
The abstract operation TriggerPromiseReactions takes arguments reactions (a List of PromiseReaction Records) and argument
(an ECMAScript language value) and returns
unused. It enqueues a new Job for each record in reactions. Each such Job processes the [[Type]] and [[Handler]] of the PromiseReaction Record, and if the [[Handler]] is not empty, calls it passing the given
argument. If the [[Handler]] is empty, the behaviour is
determined by the [[Type]]. It performs the following steps when called:
The host-defined
abstract operation HostPromiseRejectionTracker takes arguments promise (a Promise) and
operation ("reject" or "handle") and returns
unused. It allows host environments to track promise rejections.
The default implementation of HostPromiseRejectionTracker is to return unused.
Note 1
HostPromiseRejectionTracker is called in two scenarios:
When a promise is rejected without any handlers, it is called with its operation
argument set to "reject".
When a handler is added to a rejected promise for the first time, it is called with its
operation argument set to "handle".
A typical implementation of HostPromiseRejectionTracker might try to notify developers of unhandled
rejections, while also being careful to notify them if such previous notifications are later
invalidated by new handlers being attached.
Note 2
If operation is "handle", an implementation should not hold a reference
to promise in a way that would interfere with garbage collection. An implementation may
hold a reference to promise if operation is "reject", since
it is expected that rejections will be rare and not on hot code paths.
The abstract operation NewPromiseReactionJob takes arguments reaction (a PromiseReaction Record) and argument (an
ECMAScript language value) and returns a
Record with fields [[Job]] (a JobAbstract
Closure) and [[Realm]] (a Realm Record or null). It
returns a new JobAbstract
Closure that applies the appropriate handler to the incoming value, and uses the
handler's return value to resolve or reject the derived promise associated with that handler. It performs
the following steps when called:
Let job be a new JobAbstract
Closure with no parameters that captures reaction and
argument and performs the following steps when called:
NOTE: handlerRealm is never null unless the handler is
undefined. When the handler is a revoked Proxy and no ECMAScript code runs,
handlerRealm is used to create error objects.
Return the Record { [[Job]]: job, [[Realm]]:
handlerRealm }.
27.2.2.2 NewPromiseResolveThenableJob ( promiseToResolve,
thenable, then )
The abstract operation NewPromiseResolveThenableJob takes arguments promiseToResolve (a
Promise), thenable (an Object), and then (a JobCallback
Record) and returns a Record with fields [[Job]] (a JobAbstract
Closure) and [[Realm]] (a Realm Record). It performs the following steps
when called:
Let job be a new JobAbstract
Closure with no parameters that captures promiseToResolve,
thenable, and then and performs the following steps when called:
NOTE: thenRealm is never null. When then.[[Callback]] is a revoked Proxy and no code runs, thenRealm is used
to create error objects.
Return the Record { [[Job]]: job, [[Realm]]:
thenRealm }.
Note
This Job uses the supplied
thenable and its then method to resolve the given promise. This process must take place
as a Job to ensure that the
evaluation of the then method occurs after evaluation of any surrounding code has
completed.
is the initial value of the "Promise" property of the global object.
creates and initializes a new Promise when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value in an extends clause of a class definition. Subclass constructors that intend to
inherit the specified Promise behaviour must include a super call to the Promise constructor to create and
initialize the subclass instance with the internal state necessary to support the Promise and
Promise.prototype built-in methods.
27.2.3.1 Promise ( executor )
This function performs the following steps when called:
If NewTarget is undefined, throw a TypeError exception.
If IsCallable(executor) is false,
throw a TypeError exception.
Let promise be ? OrdinaryCreateFromConstructor(NewTarget,
"%Promise.prototype%", « [[PromiseState]], [[PromiseResult]], [[PromiseFulfillReactions]], [[PromiseRejectReactions]], [[PromiseIsHandled]] »).
Set promise.[[PromiseState]] to pending.
Set promise.[[PromiseResult]] to empty.
Set promise.[[PromiseFulfillReactions]] to a new empty List.
Set promise.[[PromiseRejectReactions]] to a new empty List.
The executor argument must be a function object. It is called for initiating and reporting
completion of the possibly deferred action represented by this Promise. The executor is called with
two arguments: resolve and reject. These are functions that may be used by the
executor function to report eventual completion or failure of the deferred computation.
Returning from the executor function does not mean that the deferred action has been completed but
only that the request to eventually perform the deferred action has been accepted.
The resolve function that is passed to an executor function accepts a single
argument. The executor code may eventually call the resolve function to indicate
that it wishes to resolve the associated Promise. The argument passed to the resolve
function represents the eventual value of the deferred action and can be either the actual fulfillment
value or another promise which will provide the value if it is fulfilled.
The reject function that is passed to an executor function accepts a single
argument. The executor code may eventually call the reject function to indicate
that the associated Promise is rejected and will never be fulfilled. The argument passed to the
reject function is used as the rejection value of the promise. Typically it will be an
Error object.
The resolve and reject functions passed to an executor function by the Promise constructor have the
capability to actually resolve and reject the associated promise. Subclasses may have different
constructor
behaviour that passes in customized values for resolve and reject.
This function returns a new promise which is fulfilled with an array of fulfillment values for the passed
promises, or rejects with the reason of the first passed promise that rejects. It resolves all elements of
the passed iterable to promises as it runs this algorithm.
A Promise.all resolve element function is an anonymous built-in function that is used to
resolve a specific Promise.all element. Each Promise.all resolve element
function has [[Index]], [[Values]], [[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.
When a Promise.all resolve element function is called with argument x, the
following steps are taken:
The "length" property of a Promise.all resolve element function is
1𝔽.
27.2.4.2 Promise.allSettled ( iterable )
This function returns a promise that is fulfilled with an array of promise state snapshots, but only
after all the original promises have settled, i.e. become either fulfilled or rejected. It resolves all
elements of the passed iterable to promises as it runs this algorithm.
27.2.4.2.2Promise.allSettled Resolve Element Functions
A Promise.allSettled resolve element function is an anonymous built-in function that is
used to resolve a specific Promise.allSettled element. Each Promise.allSettled
resolve element function has [[Index]], [[Values]],
[[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.
When a Promise.allSettled resolve element function is called with argument x,
the following steps are taken:
The "length" property of a Promise.allSettled resolve element function
is 1𝔽.
27.2.4.2.3Promise.allSettled Reject Element Functions
A Promise.allSettled reject element function is an anonymous built-in function that is
used to reject a specific Promise.allSettled element. Each Promise.allSettled
reject element function has [[Index]], [[Values]],
[[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.
When a Promise.allSettled reject element function is called with argument x,
the following steps are taken:
The "length" property of a Promise.allSettled reject element function
is 1𝔽.
27.2.4.3 Promise.any ( iterable )
This function returns a promise that is fulfilled by the first given promise to be fulfilled, or rejected
with an AggregateError holding the rejection reasons if all of the given promises are
rejected. It resolves all elements of the passed iterable to promises as it runs this
algorithm.
A Promise.any reject element function is an anonymous built-in function that is used to
reject a specific Promise.any element. Each Promise.any reject element
function has [[Index]], [[Errors]], [[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.
When a Promise.any reject element function is called with argument x, the
following steps are taken:
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
27.2.4.5 Promise.race ( iterable )
This function returns a new promise which is settled in the same way as the first passed promise to
settle. It resolves all elements of the passed iterable to promises as it runs this algorithm.
If the iterable argument yields no values or if none of the promises yielded by
iterable ever settle, then the pending promise returned by this method will never be
settled.
Note 2
This function expects its this value to be a constructor function that supports the
parameter conventions of the Promise constructor. It also expects that its this
value provides a resolve method.
Perform ? Call(promiseCapability.[[Reject]], undefined, « r »).
Return promiseCapability.[[Promise]].
Note
This function expects its this value to be a constructor function that supports the
parameter conventions of the Promise constructor.
27.2.4.7 Promise.resolve ( x )
This function returns either a new promise resolved with the passed argument, or the argument itself if
the argument is a promise produced by this constructor.
Promise[%Symbol.species%] is an accessor property whose set accessor function is
undefined. Its get accessor function performs the following steps when called:
Return the this value.
The value of the "name" property of this function is "get
[Symbol.species]".
Note
Promise prototype methods normally use their this value's constructor to create a
derived object. However, a subclass constructor may over-ride that default behaviour by redefining
its %Symbol.species% property.
The abstract operation PerformPromiseThen takes arguments promise (a Promise),
onFulfilled (an ECMAScript language value), and
onRejected (an ECMAScript language value) and optional
argument resultCapability (a PromiseCapability Record) and
returns an ECMAScript language value. It performs the
“then” operation on promise using onFulfilled and onRejected as its
settlement actions. If resultCapability is passed, the result is stored by updating
resultCapability's promise. If it is not passed, then PerformPromiseThen is being called by a
specification-internal operation where the result does not matter. It performs the following steps when
called:
creates and initializes a new GeneratorFunction when called as a function rather than as a constructor. Thus the function
call GeneratorFunction (…) is equivalent to the object creation expression
new GeneratorFunction (…) with the same arguments.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to
inherit the specified GeneratorFunction behaviour must include a super call to the
GeneratorFunction constructor to create and initialize subclass instances with the
internal slots necessary for built-in GeneratorFunction behaviour. All ECMAScript syntactic forms for
defining generator function
objects create direct instances of GeneratorFunction. There is no syntactic means to
create instances of GeneratorFunction subclasses.
The initial value of the %Symbol.toStringTag% property is the String value
"GeneratorFunction".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true }.
27.3.4 GeneratorFunction Instances
Every GeneratorFunction instance is an ECMAScript function object and has the internal slots listed in Table 28. The value of the
[[IsClassConstructor]] internal slot for all such instances is
false.
Each GeneratorFunction instance has the following own properties:
27.3.4.1 length
The specification for the "length" property of Function instances given in 20.2.4.1 also applies to GeneratorFunction
instances.
27.3.4.2 name
The specification for the "name" property of Function instances given in 20.2.4.2 also applies to GeneratorFunction
instances.
27.3.4.3 prototype
Whenever a GeneratorFunction instance is created another ordinary object is also created and is the
initial value of the generator function's "prototype" property. The value of the
prototype property is used to initialize the [[Prototype]] internal slot of a
newly created Generator when the generator function object is invoked using [[Call]].
This property has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]:
false }.
Note
Unlike Function instances, the object that is the value of a GeneratorFunction's
"prototype" property does not have a "constructor" property
whose value is the GeneratorFunction instance.
creates and initializes a new AsyncGeneratorFunction when called as a function rather than as a
constructor. Thus the
function call AsyncGeneratorFunction (...) is equivalent to the object creation expression
new AsyncGeneratorFunction (...) with the same arguments.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to
inherit the specified AsyncGeneratorFunction behaviour must include a super call to the
AsyncGeneratorFunction constructor to create and initialize subclass instances with the
internal slots necessary for built-in AsyncGeneratorFunction behaviour. All ECMAScript syntactic forms for
defining async generator function objects create direct instances of
AsyncGeneratorFunction. There is no syntactic means to create instances of AsyncGeneratorFunction
subclasses.
The initial value of the %Symbol.toStringTag% property is the String value
"AsyncGeneratorFunction".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true }.
27.4.4 AsyncGeneratorFunction Instances
Every AsyncGeneratorFunction instance is an ECMAScript function object and has the internal slots listed in Table 28. The value of the
[[IsClassConstructor]] internal slot for all such instances is
false.
Each AsyncGeneratorFunction instance has the following own properties:
27.4.4.1 length
The value of the "length" property is an integral Number that indicates the typical
number of arguments expected by the AsyncGeneratorFunction. However, the language permits the function to
be invoked with some other number of arguments. The behaviour of an AsyncGeneratorFunction when invoked on
a number of arguments other than the number specified by its "length" property depends
on the function.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true }.
27.4.4.2 name
The specification for the "name" property of Function instances given in 20.2.4.2 also applies to AsyncGeneratorFunction
instances.
27.4.4.3 prototype
Whenever an AsyncGeneratorFunction instance is created, another ordinary object is also created and is the
initial value of the async generator function's "prototype" property. The value of the
prototype property is used to initialize the [[Prototype]] internal slot of a
newly created AsyncGenerator when the generator function object is invoked using [[Call]].
This property has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]:
false }.
Note
Unlike function instances, the object that is the value of an AsyncGeneratorFunction's
"prototype" property does not have a "constructor" property
whose value is the AsyncGeneratorFunction instance.
Generator instances directly inherit properties from the initial value of the "prototype"
property of the generator function that created the instance. Generator instances indirectly inherit
properties from %GeneratorPrototype%.
The abstract operation GeneratorStart takes arguments generator (a Generator) and
generatorBody (a FunctionBodyParse
Node or an Abstract Closure with no parameters) and returns
unused. It performs the following steps when called:
Assert:
generator.[[GeneratorState]] is
suspended-start.
NOTE: Once a generator enters the completed state it never leaves it and
its associated execution context is never resumed. Any
execution state associated with acGenerator can be discarded at this point.
The abstract operation GeneratorValidate takes arguments generator (an ECMAScript language value) and
generatorBrand (a String or empty) and returns either a normal completion containing one of
suspended-start, suspended-yield, or
completed, or a throw completion. It
performs the following steps when called:
Resume the suspended evaluation of genContext using
NormalCompletion(value) as the result of
the operation that suspended it. Let result be the value returned by the resumed
computation.
NOTE: Once a generator enters the completed state it never leaves it and
its associated execution context is never resumed. Any
execution state associated with generator can be discarded at this point.
Resume the suspended evaluation of genContext using
abruptCompletion as the result of the operation that suspended it. Let result be
the Completion Record returned by the
resumed computation.
Resume callerContext passing NormalCompletion(iteratorResult). If
genContext is ever resumed again, let resumptionValue be the Completion Record with which it is
resumed.
The abstract operation CreateIteratorFromClosure takes arguments closure (an Abstract
Closure with no parameters), generatorBrand (a String or
empty), and generatorPrototype (an Object) and optional argument
extraSlots (a List of names of internal slots) and
returns a Generator. It performs the following steps when called:
AsyncGenerator instances directly inherit properties from the initial value of the
"prototype" property of the async generator function that created the instance.
AsyncGenerator instances indirectly inherit properties from %AsyncGeneratorPrototype%.
Records which represent requests
to resume the async generator. Except during state transitions, it is non-empty if and only if
[[AsyncGeneratorState]] is either executing or
draining-queue.
[[GeneratorBrand]]
a String or empty
A brand used to distinguish different kinds of async generators. The [[GeneratorBrand]] of async generators declared by ECMAScript source
text is always empty.
27.6.3 AsyncGenerator Abstract Operations
27.6.3.1 AsyncGeneratorRequest Records
An AsyncGeneratorRequest is a Record value used to store information
about how an async generator should be resumed and contains capabilities for fulfilling or rejecting the
corresponding promise.
The abstract operation AsyncGeneratorStart takes arguments generator (an AsyncGenerator) and
generatorBody (a FunctionBodyParse
Node or an Abstract Closure with no parameters) and returns
unused. It performs the following steps when called:
Assert:
generator.[[AsyncGeneratorState]] is
suspended-start.
The abstract operation AsyncGeneratorEnqueue takes arguments generator (an AsyncGenerator),
completion (a Completion Record), and
promiseCapability (a PromiseCapability Record) and returns
unused. It performs the following steps when called:
Let request be AsyncGeneratorRequest { [[Completion]]: completion, [[Capability]]:
promiseCapability }.
Append request to generator.[[AsyncGeneratorQueue]].
The abstract operation AsyncGeneratorCompleteStep takes arguments generator (an
AsyncGenerator), completion (a Completion Record),
and done (a Boolean) and optional argument realm (a Realm Record) and returns
unused. It performs the following steps when called:
Assert:
generator.[[AsyncGeneratorQueue]] is not empty.
Let next be the first element of generator.[[AsyncGeneratorQueue]].
Remove the first element from generator.[[AsyncGeneratorQueue]].
The abstract operation AsyncGeneratorResume takes arguments generator (an AsyncGenerator) and
completion (a Completion Record) and returns
unused. It performs the following steps when called:
Assert:
generator.[[AsyncGeneratorState]] is either
suspended-start or suspended-yield.
Let genContext be generator.[[AsyncGeneratorContext]].
Resume the suspended evaluation of genContext using
completion as the result of the operation that suspended it. Let result be the
Completion Record returned by the
resumed computation.
The abstract operation AsyncGeneratorAwaitReturn takes argument generator (an AsyncGenerator)
and returns unused. It performs the following steps when called:
Assert:
generator.[[AsyncGeneratorState]] is
draining-queue.
The abstract operation AsyncGeneratorDrainQueue takes argument generator (an AsyncGenerator)
and returns unused. It drains the generator's AsyncGeneratorQueue until it
encounters an AsyncGeneratorRequest which holds a return completion. It performs the
following steps when called:
Assert:
generator.[[AsyncGeneratorState]] is
draining-queue.
Let queue be generator.[[AsyncGeneratorQueue]].
Repeat, while queue is not empty,
Let next be the first element of queue.
Let completion be Completion(next.[[Completion]]).
The abstract operation CreateAsyncIteratorFromClosure takes arguments closure (an Abstract
Closure with no parameters), generatorBrand (a String or
empty), and generatorPrototype (an Object) and returns an
AsyncGenerator. It performs the following steps when called:
creates and initializes a new AsyncFunction when called as a function rather than as a constructor. Thus the function
call AsyncFunction(…) is equivalent to the object creation expression
new AsyncFunction(…) with the same arguments.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to
inherit the specified AsyncFunction behaviour must include a super call to the AsyncFunction
constructor to create
and initialize a subclass instance with the internal slots necessary for built-in async function
behaviour. All ECMAScript syntactic forms for defining async function objects create direct instances of
AsyncFunction. There is no syntactic means to create instances of AsyncFunction subclasses.
The initial value of the %Symbol.toStringTag% property is the String value
"AsyncFunction".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true }.
27.7.4 AsyncFunction Instances
Every AsyncFunction instance is an ECMAScript function object and has the internal slots listed in Table 28. The value of the
[[IsClassConstructor]] internal slot for all such instances is
false. AsyncFunction instances are not constructors and do not have a [[Construct]]
internal method. AsyncFunction instances do not have a prototype property as they are not constructable.
Each AsyncFunction instance has the following own properties:
27.7.4.1 length
The specification for the "length" property of Function instances given in 20.2.4.1 also applies to AsyncFunction instances.
27.7.4.2 name
The specification for the "name" property of Function instances given in 20.2.4.2 also applies to AsyncFunction instances.
The abstract operation AsyncBlockStart takes arguments promiseCapability (a PromiseCapability Record), asyncBody
(a Parse
Node or an Abstract Closure with no parameters), and
asyncContext (an execution context) and returns
unused. It performs the following steps when called:
Set the code evaluation state of asyncContext such that when evaluation is resumed for
that execution context, closure will be called
with no arguments.
Assert: result
is a normal completion with a value of
unused. The possible sources of this value are Await or, if the async function doesn't await
anything, step 2.i above.
A Module Namespace Object is a module namespace exotic object that provides runtime
property-based access to a module's exported bindings. There is no constructor function for Module Namespace Objects.
Instead, such an object is created for each module that is imported by an ImportDeclaration that contains a NameSpaceImport.
In addition to the properties specified in 10.4.6 each Module Namespace
Object has the following own property:
28.3.1 %Symbol.toStringTag%
The initial value of the %Symbol.toStringTag% property is the String value
"Module".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
29 Memory Model
The memory consistency model, or memory model, specifies the possible orderings of
Shared
Data Block events, arising via accessing TypedArray instances backed by a SharedArrayBuffer and via methods on the
Atomics object. When the program has no data races (defined below), the ordering of events appears as
sequentially consistent, i.e., as an interleaving of actions from each agent. When the program has data races, shared memory
operations may appear sequentially inconsistent. For example, programs may exhibit causality-violating behaviour
and other astonishments. These astonishments arise from compiler transforms and the design of CPUs (e.g.,
out-of-order execution and speculation). The memory model defines both the precise conditions under which a
program exhibits sequentially consistent behaviour as well as the possible values read from data races. To wit,
there is no undefined behaviour.
The memory model is defined as relational constraints on events introduced by abstract operations on
SharedArrayBuffer or by methods on the Atomics object during an evaluation.
Note
This section provides an axiomatic model on events introduced by the abstract operations on
SharedArrayBuffers. It bears stressing that the model is not expressible algorithmically, unlike the rest of
this specification. The nondeterministic introduction of events by abstract operations is the
interface between the operational semantics of ECMAScript evaluation and the axiomatic semantics of the
memory model. The semantics of these events is defined by considering graphs of all events in an evaluation.
These are neither Static Semantics nor Runtime Semantics. There is no demonstrated algorithmic
implementation, but instead a set of constraints that determine if a particular event graph is allowed or
disallowed.
29.1 Memory Model Fundamentals
Shared memory accesses (reads and writes) are divided into two groups, atomic accesses and data accesses,
defined below. Atomic accesses are sequentially consistent, i.e., there is a strict total ordering of events
agreed upon by all agents in an
agent
cluster. Non-atomic accesses do not have a strict total ordering agreed upon by all
agents, i.e., unordered.
Note 1
No orderings weaker than sequentially consistent and stronger than unordered, such as release-acquire,
are supported.
A Shared Data Block event is either a ReadSharedMemory, WriteSharedMemory, or ReadModifyWriteSharedMemory Record.
These events are introduced by abstract operations
or by methods on the Atomics object.
Some operations may also introduce Synchronize events. A Synchronize event has no fields, and exists purely to
directly constrain the permitted orderings of other events.
In addition to Shared Data
Block and Synchronize events, there are host-specific events.
Let the range of a ReadSharedMemory, WriteSharedMemory, or ReadModifyWriteSharedMemory event be the Set of
contiguous integers from its [[ByteIndex]] to [[ByteIndex]] + [[ElementSize]] - 1. Two events' ranges are equal when the events have the same [[Block]], and the ranges are element-wise equal. Two events' ranges are overlapping
when the events have the same [[Block]], the ranges are not equal and their
intersection is non-empty. Two events' ranges are disjoint when the events do not have the same [[Block]] or their ranges are neither equal nor overlapping.
Note 2
Examples of host-specific
synchronizing events that should be accounted for are: sending a SharedArrayBuffer from one agent to another (e.g., by
postMessage in a browser), starting and stopping agents, and communicating within the agent cluster via channels other than
shared memory. For a particular execution execution, those events are provided by the host via the host-synchronizes-withstrict partial order. Additionally,
hosts can add host-specific synchronizing events to
execution.[[EventList]] so as to participate in the is-agent-order-beforeRelation.
An empty candidate execution is a candidate
execution Record whose fields are empty Lists.
29.5 Abstract Operations for the Memory Model
29.5.1 EventSet ( execution )
The abstract operation EventSet takes argument execution (a candidate
execution) and returns a Set of events. It performs the following steps when called:
The abstract operation SharedDataBlockEventSet takes argument execution (a candidate
execution) and returns a Set of events. It performs the following steps when called:
The abstract operation HostEventSet takes argument execution (a candidate
execution) and returns a Set of events. It performs the following steps when called:
Let bytesModified be W.[[ModifyOp]](bytes, W.[[Payload]]).
Let byte be bytesModified[payloadIndex].
Append byte to bytesRead.
Set byteLocation to byteLocation + 1.
Return bytesRead.
Note 1
The read-modify-write modification [[ModifyOp]] is given by the function
properties on the Atomics object that introduce ReadModifyWriteSharedMemory
events.
For events E and D, E is-agent-order-before D in
execution if there is some Agent Events Recordaer in
execution.[[EventsRecords]] such that aer.[[EventList]] contains both E and D and E is before
D in List order of aer.[[EventList]].
For events R and W, R reads-from W in execution
if SharedDataBlockEventSet(execution)
contains both R and W, and reads-bytes-from(R) in execution
contains W.
For events R and W, W synchronizes-with R in
execution if Rreads-fromW in execution,
R.[[Order]] is seq-cst, W.[[Order]] is seq-cst, and R and W have
equal ranges.
For each element eventsRecord of execution.[[EventsRecords]], the following is true.
For events S and Sw, S synchronizes-with Sw in
execution if eventsRecord.[[AgentSynchronizesWith]]
contains (S, Sw).
For events E and D, E synchronizes-with D in
execution if execution.[[HostSynchronizesWith]] contains
(E, D).
Note 1
Owing to convention in memory model literature, in a candidate
executionexecution, write events synchronizes-with read events, instead
of read events synchronizes-with write events.
In a candidate executionexecution, not all
seq-cst events related by reads-from are related by synchronizes-with. Only events that
also have equal ranges are related by synchronizes-with.
Assert: The
remainder of dividing R.[[ByteIndex]] by R.[[ElementSize]] is 0.
For each event W such that Rreads-fromW in
execution and W.[[NoTear]] is
true, do
If R and W have equal ranges and there exists an event V
such that V and W have equal ranges, V.[[NoTear]] is true, W and V
are not the same Shared Data Block event, and
Rreads-fromV in execution,
then
Return false.
Return true.
Note
An event's [[NoTear]] field is true when that event was
introduced via accessing an integerTypedArray, and false when introduced via
accessing a floating point TypedArray or DataView.
Intuitively, this requirement says when a memory range is accessed in an aligned fashion via an
integerTypedArray, a single write
event on that range must "win" when in a data race with other write events with equal ranges. More
precisely, this requirement says an aligned read event cannot read a value composed of bytes from
multiple, different write events all with equal ranges. It is possible, however, for an aligned read
event to read from multiple write events with overlapping ranges.
For events E and D, E is-memory-order-before D in
execution if Ehappens-beforeD in execution.
For events R and W such that Rreads-fromW in
execution, there is no WriteSharedMemory or ReadModifyWriteSharedMemory event V
in SharedDataBlockEventSet(execution) such
that V.[[Order]] is seq-cst, W
is-memory-order-before V in execution, V is-memory-order-before
R in execution, and any of the following conditions are true.
This clause together with the forward progress guarantee on agents ensure the liveness condition that
seq-cst writes become visible to seq-cst reads with
equal range in finite
time.
While is-memory-order-before includes all events in EventSet(execution), those that are not constrained
by happens-before or synchronizes-with in
execution are allowed to occur anywhere in the order.
29.7.5 Valid Executions
A candidate
executionexecution is a valid execution (or simply an execution) if all of
the following are true.
For an execution execution and events E and D that are contained in
SharedDataBlockEventSet(execution),
E and D are in a race if the following algorithm returns
true.
For an execution execution and events E and D that are contained in
SharedDataBlockEventSet(execution),
E and D are in a data race if the following algorithm returns
true.
If E.[[Order]] is not seq-cst or
D.[[Order]] is not seq-cst, then
Return true.
If E and D have overlapping ranges, then
Return true.
Return false.
29.10 Data Race Freedom
An execution execution is data race free if there are no two events in
SharedDataBlockEventSet(execution) that are
in a data race.
A program is data race free if all its executions are data race free.
The memory
model guarantees sequential consistency of all events for data race free programs.
29.11 Shared Memory Guidelines
Note 1
The following are guidelines for ECMAScript programmers working with shared memory.
We recommend programs be kept data race free, i.e., make it so that it is impossible
for there to be concurrent non-atomic operations on the same memory location. Data race
free programs have interleaving semantics where each step in the evaluation semantics
of each agent are interleaved with
each other. For data race free programs, it is not necessary to
understand the details of the memory model. The details are unlikely to build intuition that
will help one to better write ECMAScript.
More generally, even if a program is not data race free it may have predictable behaviour, so long
as atomic operations are not involved in any data races and the operations that race all have the same
access size. The simplest way to arrange for atomics not to be involved in races is to ensure that
different memory cells are used by atomic and non-atomic operations and that atomic accesses of different
sizes are not used to access the same cells at the same time. Effectively, the program should treat shared
memory as strongly typed as much as possible. One still cannot depend on the ordering and timing of
non-atomic accesses that race, but if memory is treated as strongly typed the racing accesses will not
"tear" (bits of their values will not be mixed).
Note 2
The following are guidelines for ECMAScript implementers writing compiler transformations for programs
using shared memory.
It is desirable to allow most program transformations that are valid in a single-agent setting in a multi-agent setting, to ensure that the performance of each
agent in a multi-agent program is as good as it would be in
a single-agent setting. Frequently
these transformations are hard to judge. We outline some rules about program transformations that are
intended to be taken as normative (in that they are implied by the memory model or stronger than what the
memory model
implies) but which are likely not exhaustive. These rules are intended to apply to program transformations
that precede the introductions of the events that make up the is-agent-order-beforeRelation.
Let possible read values of a read event be the set of all values of ValueOfReadEvent
for that event across all valid executions.
Any transformation of an agent-order slice that is valid in the absence of shared memory is valid in the
presence of shared memory, with the following exceptions.
Atomics are carved in stone: Program transformations must not cause the
seq-cst events in an agent-order slice to be reordered with its
unordered operations, nor its seq-cst operations to be
reordered with each other, nor may a program transformation remove a seq-cst
operation from the is-agent-order-beforeRelation.
(In practice, the prohibition on reorderings forces a compiler to assume that every
seq-cst operation is a synchronization and included in the final is-memory-order-beforeRelation, which it would usually
have to assume anyway in the absence of inter-agent program analysis. It also forces the compiler to assume that
every call where the callee's effects on the memory-order are unknown may contain
seq-cst operations.)
Reads must be stable: Any given shared memory read must only observe a single value in an
execution.
(For example, if what is semantically a single read in the program is executed multiple times then
the program is subsequently allowed to observe only one of the values read. A transformation known as
rematerialization can violate this rule.)
Writes must be stable: All observable writes to shared memory must follow from program
semantics in an execution.
(For example, a transformation may not introduce certain observable writes, such as by using
read-modify-write operations on a larger location to write a smaller datum, writing a value to memory
that the program could not have written, or writing a just-read value back to the location it was read
from, if that location could have been overwritten by another agent after the read.)
Possible read values must be non-empty: Program transformations cannot cause the possible
read values of a shared memory read to become empty.
(Counterintuitively, this rule in effect restricts transformations on writes, because writes have
force in memory
model insofar as to be read by read events. For example, writes may be moved and
coalesced and sometimes reordered between two seq-cst operations, but the
transformation may not remove every write that updates a location; some write must be preserved.)
Examples of transformations that remain valid are: merging multiple non-atomic reads from the same
location, reordering non-atomic reads, introducing speculative non-atomic reads, merging multiple
non-atomic writes to the same location, reordering non-atomic writes to different locations, and hoisting
non-atomic reads out of loops even if that affects termination. Note in general that aliased TypedArrays make it hard to
prove that locations are different.
Note 3
The following are guidelines for ECMAScript implementers generating machine code for shared memory
accesses.
For architectures with memory models no weaker than those of ARM or Power, non-atomic stores and loads
may be compiled to bare stores and loads on the target architecture. Atomic stores and loads may be
compiled down to instructions that guarantee sequential consistency. If no such instructions exist, memory
barriers are to be employed, such as placing barriers on both sides of a bare store or load.
Read-modify-write operations may be compiled to read-modify-write instructions on the target architecture,
such as LOCK-prefixed instructions on x86, load-exclusive/store-exclusive instructions on
ARM, and load-link/store-conditional instructions on Power.
Specifically, the memory
model is intended to allow code generation as follows.
Every atomic operation in the program is assumed to be necessary.
Atomic operations are never rearranged with each other or with non-atomic operations.
Functions are always assumed to perform atomic operations.
Atomic operations are never implemented as read-modify-write operations on larger data, but as
non-lock-free atomics if the platform does not have atomic operations of the appropriate size. (We
already assume that every platform has normal memory access operations of every interesting size.)
Naive code generation uses these patterns:
Regular loads and stores compile to single load and store instructions.
Lock-free atomic loads and stores compile to a full (sequentially consistent) fence, a regular load or
store, and a full fence.
Lock-free atomic read-modify-write accesses compile to a full fence, an atomic read-modify-write
instruction sequence, and a full fence.
Non-lock-free atomics compile to a spinlock acquire, a full fence, a series of non-atomic load and
store instructions, a full fence, and a spinlock release.
That mapping is correct so long as an atomic operation on an address range does not race with a
non-atomic write or with an atomic operation of different size. However, that is all we need: the
memory model
effectively demotes the atomic operations involved in a race to non-atomic status. On the other hand, the
naive mapping is quite strong: it allows atomic operations to be used as sequentially consistent fences,
which the memory
model does not actually guarantee.
Local improvements to those basic patterns are also allowed, subject to the constraints of the memory model. For
example:
There are obvious platform-dependent improvements that remove redundant fences. For example, on x86
the fences around lock-free atomic loads and stores can always be omitted except for the fence following
a store, and no fence is needed for lock-free read-modify-write instructions, as these all use
LOCK-prefixed instructions. On many platforms there are fences of several strengths, and
weaker fences can be used in certain contexts without destroying sequential consistency.
Most modern platforms support lock-free atomics for all the data sizes required by ECMAScript atomics.
Should non-lock-free atomics be needed, the fences surrounding the body of the atomic operation can
usually be folded into the lock and unlock steps. The simplest solution for non-lock-free atomics is to
have a single lock word per SharedArrayBuffer.
There are also more complicated platform-dependent local improvements, requiring some code analysis.
For example, two back-to-back fences often have the same effect as a single fence, so if code is
generated for two atomic operations in sequence, only a single fence need separate them. On x86, even a
single fence separating atomic stores can be omitted, as the fence following a store is only needed to
separate the store from a subsequent load.
Annex B (normative) Additional ECMAScript Features
for Web Browsers
The ECMAScript language syntax and semantics defined in this annex are required when the ECMAScript host is a web browser. The content of this annex
is normative but optional if the ECMAScript host is not a web browser.
Note
This annex describes various legacy features and other characteristics of web browser ECMAScript hosts. All of the language features and
behaviours specified in this annex have one or more undesirable characteristics and in the absence of legacy
usage would be removed from this specification. However, the usage of these features by large numbers of
existing web pages means that web browsers must continue to support them. The specifications in this annex
define the requirements for interoperable implementations of these legacy features.
These features are not considered part of the core ECMAScript language. Programmers should not use or
assume the existence of these features and behaviours when writing new ECMAScript code. ECMAScript
implementations are discouraged from implementing these features unless the implementation is part of a web
browser or is required to run the same legacy ECMAScript code that web browsers encounter.
B.1 Additional Syntax
B.1.1 HTML-like Comments
The syntax and semantics of 12.4 is extended as follows except that this extension is not
allowed when parsing source text using the goal symbolModule:
The syntax of 22.2.1 is
modified and extended as follows. These changes introduce ambiguities that are broken by the ordering of
grammar productions and by contextual information. When parsing using the following grammar, each
alternative is considered only if previous production alternatives do not match.
This alternative pattern grammar and semantics only changes the syntax and semantics of BMP patterns. The
following grammar extensions include productions parameterized with the [UnicodeMode] parameter. However,
none of these extensions change the syntax of Unicode patterns recognized when parsing with the
[UnicodeMode] parameter present on the goal symbol.
Return the CharSet containing the single character \
U+005C (REVERSE SOLIDUS).
Note
This production can only be reached from the sequence \c within a
character class where it is not followed by an acceptable control character.
B.1.2.8.1 CharacterRangeOrUnion ( rer, A,
B )
The abstract operation CharacterRangeOrUnion takes arguments rer (a RegExp
Record), A (a CharSet), and B (a CharSet) and returns
a CharSet.
It performs the following steps when called:
The abstract operation ParsePattern takes arguments patternText (a
sequence of Unicode code points), u (a Boolean), and v (a Boolean). It performs the
following steps when called:
If v is true and u is true, then
Let parseResult be a List containing
one or more SyntaxError objects.
Else if v is true, then
Let parseResult be ParseText(patternText, Pattern[+UnicodeMode, +UnicodeSetsMode,
+NamedCaptureGroups]).
Else if u is true, then
Let parseResult be ParseText(patternText, Pattern[+UnicodeMode, ~UnicodeSetsMode,
+NamedCaptureGroups]).
Else,
Let parseResult be ParseText(patternText, Pattern[~UnicodeMode, ~UnicodeSetsMode,
~NamedCaptureGroups]).
This function is a property of the global object. It computes a new version of a String value in
which certain code units have been replaced by a hexadecimal escape sequence.
When replacing a code unit of numeric value less than or equal to 0x00FF, a two-digit escape sequence of
the form %xx is used. When replacing a code unit of numeric value strictly greater
than 0x00FF, a four-digit escape sequence of the form %uxxxx is used.
The encoding is partly based on the encoding described in RFC 1738, but the entire encoding specified
in this standard is described above without regard to the contents of RFC 1738. This encoding does not
reflect changes to RFC 1738 made by RFC 3986.
B.2.1.2 unescape ( string )
This function is a property of the global object. It computes a new version of a String value in
which each escape sequence of the sort that might be introduced by the escape function is
replaced with the code unit that it represents.
B.2.2 Additional Properties of the String.prototype Object
B.2.2.1 String.prototype.substr ( start, length )
This method returns a substring of the result of converting the
this value to a String, starting from index start and running for
length code units (or through the end of the String if length is
undefined). If start is negative, it is treated as sourceLength + start where sourceLength is the
length of the String. The result is a String value,
not a String object.
Return the substring
of S from intStart to intEnd.
Note
This method is intentionally generic; it does not require that its this value be a
String object. Therefore it can be transferred to other kinds of objects for use as a method.
B.2.2.2 String.prototype.anchor ( name )
This method performs the following steps when called:
Let escapedV be the String value that is the same as V except that each
occurrence of the code unit 0x0022 (QUOTATION MARK) in V has been replaced with the
six code unit sequence """.
The property "trimStart" is preferred. The "trimLeft" property
is provided principally for compatibility with old code. It is recommended that the
"trimStart" property be used in new ECMAScript code.
The initial value of the "trimLeft" property is %String.prototype.trimStart%, defined
in 22.1.3.34.
B.2.2.16 String.prototype.trimRight ( )
Note
The property "trimEnd" is preferred. The "trimRight" property
is provided principally for compatibility with old code. It is recommended that the
"trimEnd" property be used in new ECMAScript code.
The initial value of the "trimRight" property is %String.prototype.trimEnd%, defined
in 22.1.3.33.
B.2.3 Additional Properties of the Date.prototype Object
B.2.3.1 Date.prototype.getYear ( )
Note
The getFullYear method is preferred for nearly all purposes, because it avoids the “year
2000 problem.”
This method performs the following steps when called:
This method completely reinitializes the this value RegExp with a new pattern and
flags. An implementation may interpret use of this method as an assertion that the resulting RegExp
object will be used multiple times and hence is a candidate for extra optimization.
B.3 Other Additional Features
B.3.1 Labelled Function Declarations
Prior to ECMAScript 2015, the specification of LabelledStatement did not allow for the association of a
statement label with a FunctionDeclaration. However, a labelled FunctionDeclaration was an allowable
extension for non-strict
code and most browser-hosted ECMAScript implementations supported that extension. In
ECMAScript 2015 and later, the grammar production for LabelledStatement permits use of FunctionDeclaration as a LabelledItem but 14.13.1 includes an Early
Error rule that produces a Syntax Error if that occurs. That rule is modified with the addition of the
highlighted text:
B.3.2 Block-Level Function Declarations Web Legacy Compatibility Semantics
Prior to ECMAScript 2015, the ECMAScript specification did not define the occurrence of a FunctionDeclaration as an element of a
Block statement's StatementList. However, support for that form of FunctionDeclaration was an allowable
extension and most browser-hosted ECMAScript implementations permitted them. Unfortunately, the semantics of
such declarations differ among those implementations. Because of these semantic differences, existing web
ECMAScript source
text that uses Block level
function declarations is only portable among browser implementations if the usage only depends upon the
semantic intersection of all of the browser implementations for such declarations. The following are the use
cases that fall within that intersection semantics:
A function is declared and only referenced within a single block.
One or more FunctionDeclarations whose BindingIdentifier is the name
f occur within the function code of an enclosing function g and that declaration
is nested within a Block.
No other declaration of f that is not a var declaration occurs within the
function code of g.
A function is declared and possibly used within a single Block but also referenced by an inner function definition that is
not contained within that same Block.
One or more FunctionDeclarations whose BindingIdentifier is the name
f occur within the function code of an enclosing function g and that declaration
is nested within a Block.
No other declaration of f that is not a var declaration occurs within the
function code of g.
There is at least one occurrence of f as an IdentifierReference within another function
h that is nested within g and no other declaration of f shadows the
references to f from within h.
All invocations of h occur after the declaration of f has been evaluated.
A function is declared and possibly used within a single block but also referenced within subsequent
blocks.
One or more FunctionDeclaration whose BindingIdentifier is the name f occur
within the function code of an enclosing function g and that declaration is nested within a
Block.
No other declaration of f that is not a var declaration occurs within the
function code of g.
There is at least one occurrence of f as an IdentifierReference within the function code of
g that lexically follows the Block containing the declaration of f.
The first use case is interoperable with the semantics of Block level function declarations provided by ECMAScript 2015. Any
pre-existing ECMAScript source
text that employs that use case will operate using the Block level function declarations
semantics defined by clauses 10, 14, and 15.
ECMAScript 2015 interoperability for the second and third use cases requires the following extensions to
the clause 10, clause 15, clause 19.2.1 and clause 16.1.7 semantics.
If an ECMAScript implementation has a mechanism for reporting diagnostic warning messages, a warning should
be produced when code contains a FunctionDeclaration for which these compatibility
semantics are applied and introduce observable differences from non-compatibility semantics. For example, if
a var binding is not introduced because its introduction would create an early error, a warning message should not be
produced.
B.3.2.1 Changes to FunctionDeclarationInstantiation
NOTE: A var binding for F is only instantiated here if it is neither a
VarDeclaredName, the name of a formal parameter, or another FunctionDeclaration.
If instantiatedVarNames does not contain F and F is not
"arguments", then
It is a Syntax Error if the LexicallyDeclaredNames of StatementList contains any duplicate
entries, unless IsStrict(this production) is false and the
duplicate entries are only bound by FunctionDeclarations.
It is a Syntax Error if the LexicallyDeclaredNames of CaseBlock contains any duplicate entries,
unless IsStrict(this
production) is false and the duplicate entries are only bound by
FunctionDeclarations.
The Block of a Catch clause may contain var declarations that bind a
name that is also bound by the CatchParameter. At runtime, such bindings are instantiated
in the VariableDeclarationEnvironment. They do not shadow the same-named bindings introduced by the
CatchParameter and hence the Initializer for such var
declarations will assign to the corresponding catch parameter rather than the var binding.
This modified behaviour also applies to var and function declarations introduced
by direct eval calls contained
within the Block of a Catch clause. This change is accomplished by modifying the algorithm of
19.2.1.3 as follows:
Objects with an [[IsHTMLDDA]] internal slot are never created by this
specification. However, the document.all
object in web browsers is a host-definedexotic object with this slot that exists for web compatibility
purposes. There are no other known examples of this type of object and implementations should not create
any with the exception of document.all.
B.3.9 Runtime Errors for Function Call Assignment Targets
When a function call (13.3.6) is used as an assignment target in non-strict code, instead
of producing an early
error, a ReferenceError exception is thrown during evaluation of the
assignment.
Note
When the assignment target is the LeftHandSideExpression of an AssignmentExpression, the assignment
operator must be = or an AssignmentOperator; in particular, the allowance here
does not apply to the logical assignment operators (??=, &&=,
||=).
Assignment to an undeclared identifier or otherwise unresolvable reference does not create a property in the
global object.
When a simple assignment occurs within strict mode code, its LeftHandSideExpression must not evaluate to an
unresolvable Reference. If it does a ReferenceError exception is thrown (6.2.5.6). The LeftHandSideExpression also may not be a
reference to a data
property with the attribute value { [[Writable]]:
false }, to an accessor property with the attribute value { [[Set]]: undefined }, nor to a non-existent property of an object
whose [[Extensible]] internal slot is false. In these cases a
TypeError exception is thrown (13.15).
Arguments objects for strict
functions do not dynamically share their array-indexed property values with the corresponding formal parameter
bindings of their functions. (10.4.4).
For strict
functions, if an arguments object is created the binding of the local identifier
arguments to the arguments object is immutable and hence may not be the target of an assignment
expression. (10.2.11).
Strict mode eval code cannot instantiate variables or functions in the variable environment of the caller to
eval. Instead, a new variable environment is created and that environment is used for declaration binding
instantiation for the eval code (19.2.1).
If this is evaluated within strict mode code, then the this value is
not coerced to an object. A this value of either undefined or
null is not converted to the global object and primitive values are not converted to wrapper
objects. The this value passed via a function call (including calls made using
Function.prototype.apply and Function.prototype.call) do not coerce the passed
this value to an object (10.2.1.2, 20.2.3.1, 20.2.3.3).
When a delete operator occurs within strict mode code, a SyntaxError is thrown
if its UnaryExpression is a direct
reference to a variable, function argument, or function name (13.5.1.1).
When a delete operator occurs within strict mode code, a TypeError is thrown if
the property to be deleted has the attribute { [[Configurable]]:
false } or otherwise cannot be deleted (13.5.1.2).
An implementation may not extend, beyond that defined in this specification, the meanings within strict functions of
properties named "caller" or "arguments" of function instances.
Preparation steps before, and cleanup steps after, invocation of JobAbstract Closures. See 9.5.
D.5 Internal Methods of Exotic Objects
Any of the essential internal methods in Table 4 for any exotic object not specified within this
specification.
D.6 Built-in Objects and Methods
Any built-in objects and methods not defined within this specification, except as restricted in 17.1.
Annex E (informative) Corrections and
Clarifications in ECMAScript 2015 with Possible Compatibility Impact
9.1.1.4.14-9.1.1.4.17 Edition 5 and 5.1 used a
property existence test to determine whether a global object property corresponding to a new global declaration
already existed. ECMAScript 2015 uses an own property existence test. This corresponds to what has been most
commonly implemented by web browsers.
10.4.2.1: The 5th
Edition moved the capture of the current array length prior to the integer conversion of the array index or new length value. However, the captured length value could
become invalid if the conversion process has the side-effect of changing the array length. ECMAScript 2015
specifies that the current array length must be captured after the possible occurrence of such side-effects.
21.4.1.31: Previous
editions permitted the TimeClip
abstract operation to return either +0𝔽 or -0𝔽 as
the representation of a 0 time value. ECMAScript 2015 specifies that
+0𝔽 always returned. This means that for ECMAScript 2015 the time
value of a Date is never observably -0𝔽 and methods that
return time values never return
-0𝔽.
21.4.1.32: If a UTC offset representation is not present,
the local time zone is used. Edition 5.1 incorrectly stated that a missing time zone should be interpreted as
"z".
21.4.4.36: If the year cannot be represented using the
Date Time String Format specified in 21.4.1.32 a RangeError exception is thrown. Previous
editions did not specify the behaviour for that case.
21.4.4.41: Previous editions did not specify the value
returned by Date.prototype.toString when the time value is NaN.
ECMAScript 2015 specifies the result to be the String value "Invalid Date".
22.2.4.1, 22.2.6.13.1: Any LineTerminator code points in
the value of the "source" property of a RegExp instance must be expressed using an escape
sequence. Edition 5.1 only required the escaping of /.
22.2.6.8, 22.2.6.11: In previous editions, the
specifications for String.prototype.match and String.prototype.replace was incorrect
for cases where the pattern argument was a RegExp value whose global flag is set. The previous
specifications stated that for each attempt to match the pattern, if lastIndex did not change, it
should be incremented by 1. The correct behaviour is that lastIndex should be incremented by 1 only
if the pattern matched the empty String.
23.1.3.30: Previous editions did not specify how a
NaN value returned by a comparator was interpreted by
Array.prototype.sort. ECMAScript 2015 specifies that such as value is treated as if
+0𝔽 was returned from the comparator. ECMAScript 2015 also specifies
that ToNumber is applied to the
result returned by a comparator. In previous editions, the effect of a comparator result
that is not a Number value was implementation-defined. In practice, implementations call
ToNumber.
Annex F (informative) Additions and Changes That
Introduce Incompatibilities with Prior Editions
6.2.5: In ECMAScript 2015, Function calls are
not allowed to return a Reference Record.
9.3: In ECMAScript
2018, Template objects are canonicalized based on Parse Node (source location), instead of across all occurrences
of that template literal or tagged template in a Realm in previous editions.
12.2: In ECMAScript
2016, Unicode 8.0.0 or higher is mandated, as opposed to ECMAScript 2015 which mandated Unicode 5.1. In
particular, this caused U+180E MONGOLIAN VOWEL SEPARATOR, which was in the Space_Separator
(Zs) category and thus treated as whitespace in ECMAScript 2015, to be moved to the
Format (Cf) category (as of Unicode 6.3.0). This causes whitespace-sensitive methods
to behave differently. For example, "\u180E".trim().length was 0 in previous editions,
but 1 in ECMAScript 2016 and later. Additionally, ECMAScript 2017 mandated always using the latest
version of the Unicode Standard.
12.7:
In ECMAScript 2015, the valid code points for an IdentifierName are specified in terms of the Unicode properties
“ID_Start” and “ID_Continue”. In previous editions, the valid IdentifierName or Identifier code points were specified by enumerating various Unicode
code point categories.
12.10.1: In ECMAScript 2015, Automatic
Semicolon Insertion adds a semicolon at the end of a do-while statement if the semicolon is missing. This change
aligns the specification with the actual behaviour of most existing implementations.
13.2.5.1: In ECMAScript 2015, it
is no longer an early error
to have duplicate property names in Object Initializers.
13.15.1: In ECMAScript 2015,
strict mode
code containing an assignment to an immutable binding such as the function name of a FunctionExpression does not produce an
early error. Instead it
produces a runtime error.
14.2: In ECMAScript 2015, a
StatementList beginning with the token let
followed by the input elements LineTerminator then Identifier is the start of a LexicalDeclaration. In previous editions, automatic semicolon
insertion would always insert a semicolon before the Identifier input element.
14.7: In ECMAScript 2015, if the ( token of a
for statement is immediately followed by the token sequence let [ then the let is
treated as the start of a LexicalDeclaration. In previous editions such a token sequence
would be the start of an Expression.
14.7: In ECMAScript 2015, if the ( token of a for-in
statement is immediately followed by the token sequence let [ then the let is treated
as the start of a ForDeclaration. In
previous editions such a token sequence would be the start of an LeftHandSideExpression.
14.7: Prior to ECMAScript 2015, an initialization expression
could appear as part of the VariableDeclaration that precedes the inkeyword. In ECMAScript 2015, the ForBinding in that same position does not allow the
occurrence of such an initializer. In ECMAScript 2017, such an initializer is permitted only in non-strict code.
14.15: In
ECMAScript 2015, it is an early
error for a Catch clause to
contain a var declaration for the same Identifier that appears as the Catch clause parameter. In previous editions, such a variable declaration
would be instantiated in the enclosing variable environment but the declaration's Initializer value would be assigned to the Catch parameter.
14.15, 19.2.1.3: In ECMAScript 2015, a runtime
SyntaxError is thrown if a Catch
clause evaluates a non-strict direct eval whose eval code includes a var or
FunctionDeclaration declaration that binds the same Identifier that appears as the Catch clause parameter.
15.4.5 In ECMAScript 2015, the
function objects
that are created as the values of the [[Get]] or [[Set]]
attribute of accessor
properties in an ObjectLiteral are not constructor functions and they do not have a
"prototype" own property. In the previous edition, they were constructors and had a "prototype"
property.
20.1.2.6: In
ECMAScript 2015, if the argument to Object.freeze is not an object it is treated as if it was a
non-extensible ordinary
object with no own properties. In the previous edition, a non-object argument always causes a
TypeError to be thrown.
20.1.2.8: In ECMAScript 2015, if the argument to
Object.getOwnPropertyDescriptor is not an object an attempt is made to coerce the argument using
ToObject. If the coercion is
successful the result is used in place of the original argument value. In the previous edition, a non-object
argument always causes a TypeError to be thrown.
20.1.2.10: In ECMAScript 2015, if the argument to
Object.getOwnPropertyNames is not an object an attempt is made to coerce the argument using
ToObject. If the coercion is
successful the result is used in place of the original argument value. In the previous edition, a non-object
argument always causes a TypeError to be thrown.
20.1.2.12: In ECMAScript 2015, if the argument to
Object.getPrototypeOf is not an object an attempt is made to coerce the argument using ToObject. If the coercion is successful
the result is used in place of the original argument value. In the previous edition, a non-object argument
always causes a TypeError to be thrown.
20.1.2.16: In ECMAScript 2015, if the argument to
Object.isExtensible is not an object it is treated as if it was a non-extensible ordinary object with no own
properties. In the previous edition, a non-object argument always causes a TypeError to be
thrown.
20.1.2.17: In
ECMAScript 2015, if the argument to Object.isFrozen is not an object it is treated as if it was a
non-extensible ordinary
object with no own properties. In the previous edition, a non-object argument always causes a
TypeError to be thrown.
20.1.2.18: In
ECMAScript 2015, if the argument to Object.isSealed is not an object it is treated as if it was a
non-extensible ordinary
object with no own properties. In the previous edition, a non-object argument always causes a
TypeError to be thrown.
20.1.2.19: In
ECMAScript 2015, if the argument to Object.keys is not an object an attempt is made to coerce the
argument using ToObject. If the
coercion is successful the result is used in place of the original argument value. In the previous edition, a
non-object argument always causes a TypeError to be thrown.
20.1.2.20: In ECMAScript 2015, if the argument to
Object.preventExtensions is not an object it is treated as if it was a non-extensible ordinary object with no own
properties. In the previous edition, a non-object argument always causes a TypeError to be
thrown.
20.1.2.22: In
ECMAScript 2015, if the argument to Object.seal is not an object it is treated as if it was a
non-extensible ordinary
object with no own properties. In the previous edition, a non-object argument always causes a
TypeError to be thrown.
20.2.3.2: In ECMAScript 2015, the [[Prototype]] internal slot of a bound function is set to the [[GetPrototypeOf]] value of its target function. In the previous edition, [[Prototype]] was always set to %Function.prototype%.
20.2.4.1: In ECMAScript 2015, the
"length" property of function instances is configurable. In previous editions it was
non-configurable.
21.4.4 In ECMAScript 2015, the Date prototype object is not a Date
instance. In previous editions it was a Date instance whose TimeValue was NaN.
22.1.3.12 In ECMAScript 2015, the
String.prototype.localeCompare function must treat Strings that are canonically equivalent
according to the Unicode Standard as being identical. In previous editions implementations were permitted to
ignore canonical equivalence and could instead use a bit-wise comparison.
22.1.3.28 and 22.1.3.30 In ECMAScript 2015, lowercase/upper
conversion processing operates on code points. In previous editions such the conversion processing was only
applied to individual code units. The only affected code points are those in the Deseret block of Unicode.
22.1.3.32 In ECMAScript 2015, the
String.prototype.trim method is defined to recognize white space code points that may exist outside
of the Unicode BMP. However, as of Unicode 7 no such code points are defined. In previous editions such code
points would not have been recognized as white space.
22.2.4.1 In ECMAScript 2015, If the pattern
argument is a RegExp instance and the flags argument is not undefined, a new
RegExp instance is created just like pattern except that pattern's flags are replaced by
the argument flags. In previous editions a TypeError exception was thrown when
pattern was a RegExp instance and flags was not undefined.
22.2.6 In ECMAScript 2015, the
RegExp prototype object is not a RegExp
instance. In previous editions it was a RegExp instance whose pattern is the empty String.
25.4.15: In
ECMAScript 2019, Atomics.wake has been renamed to Atomics.notify to prevent confusion
with Atomics.wait.
27.1.6.4, 27.6.3.6: In
ECMAScript 2019, the number of Jobs enqueued
by await was reduced, which could create an observable difference in resolution order between a
then() call and an await expression.
Bibliography
IEEE 754-2019: IEEE Standard for Floating-Point Arithmetic. Institute of
Electrical and Electronic Engineers, New York (2019)
Note
There are no normative changes between IEEE 754-2008 and IEEE 754-2019 that affect the ECMA-262
specification.
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